Why don't I ever feel this "tugging" even when my arm is fully straightened (which is more "stretched" than it ever is while playing pool)? Why don't I have to hold my arm straight? If you're actually feeling anything, I'm guessing it's the gravity you mention.
But I kinda like the visualization, even though I question its physiological reality.
pj
chgo
It is a physiological reality and has been observed in countless subjects in countless studies.
I tried a couple of times to explain it without giving away the specifics I charge for and couldn't do it adequately with just general statements. However, this is a familiar concept to golf instructors who deal with any of the 'physics-perfect' swings so will use that to illustrate.
Specifically regarding the transition, the vast majority of great players have their club move at the pace of gravity from the top as they transition. So what gets things going is a slight 'fall'. When dealing with any swing that is 'physics-perfect' (ish) we are really talking about using forces that repeat. So in golf you have 2 options: gravity or torque. There are many ways to produce the latter but the way I'm referring to is a kind of passive return to rest torque that doesn't require any additional impulse to make it unwind. The idea is that of winding up till we reach range of motion (ROM) maximums and then releasing the stored energy of that coil. What happens when you max out the ROM of a joint is that the tendons of that joint get stretched tight and do not want to go beyond that point. So the club coming around the body loads these angles if you will and produces these stretches. Then, with the entire kinetic chain stretched taut without any slack spots, a move in the footwork or hips or really anywhere down the line on the chain causes the entire thing to start unwinding. No extra added impulse required. An old Masters champion described an ideal swing this way adding this summary for his students regarding power in the golf swing: "The more you put in, the less you get out". Meaning, if you just coil it, release it, and let the dominoes fall from there, a very powerful swing will be produced without you adding a damn thing to it. This type of stroke can be applied to cue sports as well tho the specifics of which dominoes fall and how change.
So for snooker, this same concept of getting all the slack out and getting the entire chain taut allows for a noticeable tension to be built up and it can easily be strong enough to be felt across the shoulder joint. When a backswing forces the stretch farther, more tension builds up and the normal state demands to be restored. This is exactly what happens when you let go of the elastic. This gets things going smoothly and on plane since we didn't introduce any new impulse which can pull a cue/golf club/whatever off the plane it is swinging on. The zip depends on timing of when and how force is added and as alluded to earlier there are many ways to do this.
this want to return to that shape. cant be very forcefull
surely not enough to get a cue going at the speed necessary
and as you know the stretch on the tendon will contribute close to nothing in the "desire to return to that shape"
As mentioned, this works hand in hand with gravity and the cue wanting to return to its rest position (address position for most players). In a loose arm, you can feel this stretch/tug. While I think you would be surprised how much force can be exerted by muscles to resist the overstretch, you are right we aren't cranking it like maniacs while cueing and getting anywhere near those limits so the forces are pretty subtle. That said, you don't need a ton of power to get the cue accelerating smoothly. And really, it is like a first domino in a line of dominoes. This little tug and gravity provide more than enough force to get the cue going and start the dominoes falling that will eventually produce a very powerful stroke.
To circle all the way back garczar's point, this discussion is a dive into the minutia of a
type of throw. Most strokes are very similar to and can be modeled as a simple throw. Getting the slack out so that one part can tug uninterruptedly on another part throughout a motion is key for seamless force transmission. As soon as you have slack, you have compensatory tightening to cover slack spots and a loss of the swing plane (aka the yips). This seamless force transmission is the real value of this elastic stretched feel. Tho it does contribute some of the force production as well.
You're welcome 
