Definition - Primer
I always want people to talk using "standard" terms and definitions. People can write and talk with definitions and use it as a language that everyone understands. When a customer talks to each other or to me they will use definitions that control the explanation of a problem. This is just a part of "Controlling" the tuner by the way they move about and correspond regarding clutching. This is how a tuner can talk with other people who are novice tuners or non-tuners. When people talk with terms, standards are used so there will be accurate descriptions of the thing by its kind and difference to the exclusion from all others.

The definitions below are of a variety from my "in progress: theory book "Increase your clutching IQ"

Ability: (primary) active power, or power to perform, the power to do certain things.

Actual mph: the observed mph at full throttle below theoretical mph.

Many times I see confusion regarding the word "Aggressive" when it comes to clutching.
People relate this word when they talk of how the clutches shift.
The clutch uses more of the meat of the torque curve from the engine, shifting harder with lower rpms.
When you quickly cycle the throttle, the engine is not revvy, the sled responds with greater movement from your throttle position.
You alter the feeling of the sled with greater action in the bottom end of the midrange when pressing the throttle.
Aggressive is: (primary) the clutch exerts more forceful energy using less engine rpm.

specifying what you observe, or specifying the capacity of the system while the sled is in motion; usually under full throttle...
You read scenarios like: weak/strong
*weak...come around the corner, pressed the throttle and the engine would hit 500 rpms low and slowly accelerate.
strong...come around the corner, pressed the throttle and the engine rpm quickly accelerates to 8t 8 grand.
*weak...sled accelerates hard to top end but as the snow load increases the engine rpms starts to drift low.
strong...accelerates hard to top end like my buddies sled except when the snow gets deep, my engine rpms stays at rated.
*weak...good acceleration but if i flip the throttle the engine comes back in at low rpms.
strong...good acceleration and when i blip the throttle, bam the engine hits the right rpms.
*weak...while climbing my rpms are 1000 rpms low.
strong...while climbing my rpms stay rock steady.
*weak...when I land after a drift the rpms come in low and won't recover till i slow down.
strong...when I land after a drift, the rpms come in low but recover while I maintain full throttle.

IF there are opinions of backshift using adjectives like "strong" or "weak", example....."Strong backshift" or "weak backshift"
THEN Backshift is: the ability for the engine to accelerate back to rated rpms, as the load changes.

Backshift: n. (secondary) the drive belt travelling back up to the rim of the secondary clutch.

Bottom End: (sled speed); the start of the upshift, beginning mph's. The first 1/3rd of the upshift.

Capacity: (secondary) the ability to receive the engine power output measured against the ground input.

Engagement: The primary sliding sheave embarks to engage with the drive belt.

Finish force:  (primary & secondary spring) the amount of lbs. force when the clutch is at full shift.

Full shift: complete open position of the primary clutch.

Holding Power: The ability to maintain a certain track speed over distance as load increases.

Midrange: (sled speed); the middle 2/3 of the upshift and mph capacity.

Overdrive: the primary clutch position drives the secondary clutch speed higher than the primary clutch speed.

Quotes Aaen: You only have two reference points when you work with a transmission. You have to compare engine speed (rpm) and vehicle speed (mph) with your gearing to know what ratio you are in.
The advantage of the snowmobile transmission is its ability to hold the engine rpm at the power curve (rated rpm) while it continually changes ratios.

Quotes BRP pulley development: The goal of clutch calibration is to keep the engine, at full throttle, at its peak power RPM and at the same time to select the highest possible gear ratio dictated by the load on the drive axle. The speed diagram illustrates what the goal of good clutch calibration is.

Summation; the ideal shift would be for the engine to be at its rated rpms from low ratio to high ratio. If the rpms vary under wide open throttle, then the shift is either over the rated rpm line or under the rated rpm line.
Overshift is tied to under revving. Under shift is tied over revving.
IF the ratio change "duration" is too small, like the primary clutch won't upshift,(too low of helix angle/too high of secondary spring forces)
THEN engine speed flares into over rev.

IF the ratio change "duration" is too large, like the primary clutch upshift ‘s too easily, (too large of helix angle/too low of secondary spring forces)
THEN the engine speed lowers into under rev.(Overshift)

Could all the information be overlapped and connected to say that...
Over shift: While accelerating at wide open throttle;
a. (clutch up shift) shifting the clutches out faster than the secondary clutch’s capacity, revealing low rpms.
b. (engine speed) A variation of low rpm caused by a surge of increased secondary load against the primary load.
c. (time) accumulated time accelerating at less than rated rpms.

Because of Aaen's reference point of having a straight rpm from low ratio to high ratio, then it could be thought as over shift is a separate detail from under shift.
a. b. c. is suitable for the definition of Overshift.
Overshift is: The primary clutch shifts up faster than secondary clutch’s capacity, revealing low rpms.
1. Frequently observed when a track spins for some distance from a start, then hooks up to the ground and the engine rpm will drift lower than rated.
2. Observed when accelerating up a hill/load and quickly one feel’s the sled acceleration slow quickly, then rpms drift low by several hundred.

Push: (primary clutch) to press against the belt with force; we push the secondary open by force applied inside the primary.

Rated Rpms: (engine speed) Continuous highest hp rpms.
1. Stock 800R engine has a continuous highest hp rpm at 8150.
2. This modified engine with twin pipes has a continuous highest hp rpm at 10250.
3. This 920 big bore has a continuous highest hp rpm at 7400.
Rpms drifting higher: engine rpms that increase above rated rpms while at full throttle
Rpms drifting lower: engine rpms that reduce away from rated rpms while at full throttle.
Shift point: (full throttle) when the rpm stops increasing at the engine’s rated rpms.
1. Low “shift point”, look at Undershift.
2. High “shift point’, look at Overshift.

Shift-out-point: (full throttle) as the rpm of the engine increases, it is the rpm that the primary starts to upshift again.
Knowing where to calibrate this point is where your skills come in handy for a practical application. 

Example (low power to weight ratio) 440 race sleds converted to trail you want the shift-out-point as close as possible to rated rpms.  You are running through the bumps cycling the throttle on and off a lot and need the power to get the sled clear of bumps, having the shift-out-point closest to the power peak allows you to achieve the highest surge of power to accelerate when you hit the throttle.

Example (high power to weight ratio) my friend Per Bostrom of Speed-Technology in Sweden makes a 920cc big bore with 125lbs torque at 7400 rpms.  Per Writes) it was the last weekend of riding the Speed-tech Summit as 920cc and that engine is too insane. I can barely ride in bump conditions, rough and narrow terrain. The torque is too much to handle when needing the extra sensitivity and smooth throttle.  The shift-out-point in the clutch is low rpm and I drive only at 1/4 throttle or less, I mean the engine have 100 lbs torque at 5500 rpm.  I've started to build a new engine, an 860cc. using big bore pipes. Should give pretty much same hp as 920cc engine but at much higher rpms, giving me the option to ride with low hp/torque at low rpms and high bhp at rated rpms ( somewhere around 8200rpm ). More throttle movement when driving if needed.

Example (shift-out-point too high) as I was clutch tuning an 800 trail sled, I kept reducing the start angle of the helix.  As I drive through the bumps, chopping the throttle on and off, the sled delay’s to accelerate hard.  When you press the throttle to the bar the engine hits its rated 8200 however the sudden surge of acceleration is not there.  With the current helix angle, it’s causing me to miss the “meat” of the torque curve of the 800.  As I returned and started to take the secondary spring start force and lower it, I could feel the sled start to pull harder again as I chop the throttle off and on through the bumps.  Eventually I chose a secondary spring with a start force that was low enough to give me a strong pull on the handlebars.

Example (shift-out-point too low) as I was clutching a twin piped 860 trail sled rated at 8600 rpms; I kept reducing the start angle of the helix.  As I drive through the bumps, chopping the throttle on and off, the sled delay’s to accelerate hard was becoming less and less.  When you press the throttle to the bar the engine hits 8300 with a sudden surge then drifts up to 8600.  With the current helix angle, it’s causing the shift-out-point to be too low.  I removed the 200/300 secondary spring and raised the start force to a 231/300.  I returned to the same bumpy trail running the sled and immediately noticed that the engine would hit 8500 and climb to 8600, the sled pulled extremely hard when chopping the throttle off and on through the bumps.  Through measuring the rpms at full throttle, I chose a secondary spring with a start force that was high enough to reveal a best shift-out-point  and strong pull on the handlebars.

Spring "Rate" & "Forces" - Definition
Here is my top pet peeve. When it comes to springs and their forces, I always hear or read the word "rate" when the word “Force” should be used. Here people describe a quantity of a spring and are not referring right to the element of the spring.
Rate is: the fixed ratio between two things.
In the case of a spring, rate is the quantity lbs measured per inch of change. You compress the spring it takes "x" amount lbs of force (per inch) to change the height of the spring. The starting point is the installed height and the finish point is the full compressed height. The installed height is at rest or the engagement or beginning of the shift. The full compressed height is at full shift.
Force is: the strength or energy exerted or brought to bear to hold something in position.
For example a 170-300 primary spring. The spring needs 170 lbs of force to keep it at installed height and must add enough force of up to 300 lbs to change its position to full compressed height..
***The spring does not have 170 lbs "rate" at the beginning of the shift.
The spring at installation is compressed 1.25" and requires 170 lbs of force to keep it that position.
The spring at full shift is compressed to 2.5" and requires 300 lbs of force to keep it that position.
To figure out the rate of the spring you do a simple calculation.
i] 300 lbs - 170 lbs = 130 lbs
ii] 2.5" - 1.25" = 1.25" of distance travel between engagement and full shift.
130 lbs / 1.25" distance = 104 lbs/inch of rate.
From installation height of 1.25" then you want to compress it just 1" more, you would have to add another 104 lbs on top of the 170 lbs to make the spring compressed to 2.25". To keep the spring compressed at 2.25" that would mean you did 170 + 104 = 274 lbs to keep it in that position.

Getting back to my pet peeve about "Force" and "Rate" example: 170-300 spring.
Question; I have too much rpms on top end, should I change to a softer spring, one with less finish rate in the primary?
Answer; the idea given here is to change from 300 and go with something lower. For lack of better numbers lets say "260".
Might want to change to a 170-260 from a 170-300.

Now that you know the definition of rate, what do you think when you read this quote where the word "rate" is used? Right idea, however not using the correct term. I'd like to correct the question by having the word rate exchanged with "force".
New question; Change to a softer spring, one with less finish "force" in the primary. Softer primary spring final force has faster up shift.

Spring force: the amount of weight needed to compress the spring to a certain position.

Start force:  (primary & secondary spring) the value of the spring in lbs. force when it is installed.

Start of shift:  The primary clutch is engaged on the belt and sets out accelerate it.

Theoretical mph: Pertaining to the maximum peak mph from the calculation of
(Engine speed x sprocket pitch dia.) / (shift ratio x gear ratio x 336)

Top End:  (sled speed) the last 1/3 of the upshift and mph capacity.  Good top end would be a peak mph satisfactory for your requirements. 

Straight shift: the rpms maintaining a certain rpm from bottom end to top end.

Track Speed: (coined by owners of mountain~longtrack sleds, but may include short track sleds) the constant or near constant speed in mph that the track is rotating while under full throttle, heavy loads. (Climbing or flat deep types of different snow)

Undershift: Over revving, engine speed higher than rated rpms.

Upshift:  (Primary) The drive belt being pushed up to the rim of the primary clutch

Upshifting:  (Primary) The drive belt travelling up to the rim of the primary clutch.