View attachment 941362
Not only muscles but bone and tendons, too (
from Shortstop's video):
The muscles and tendons of the human arm allow a wide range of motion most of which are of no benefit to shooting pool. Your triceps are connected to the ulna bone for extending the forearm. That’s the backstroke.
Your biceps are connected to the radius bone for contracting the forearm.
In the cue delivery, the forearm muscles assist a bit, but mostly serve to rotate, that's suppinate and pronate, the forearm as well as operate the tendons that control the fingers and make your cue stick not go straight.
How?
First between stretching and shortening, your tendons add energy — like springs. The faster you lengthen your tendons — that's the faster your backstroke is — and the faster the transition, the more intense is this reaction.
Now, this unwanted and unneeded energy, called amortization, is great if you're swinging an axe or a baseball bat or punching or, interestingly, breaking a rack of ten Ball, but on your pool shots, it causes a jerky and crooked stroke.
Second, is a thing called muscle recruitment when your brain knows you're about to rapidly contract a muscle. It begins recruiting those muscles before the action takes place. A fast backstroke recruits your forearm muscles such that you cannot help but engage them on the follow through causing inconsistent delivery speed and direction.
The more efficient and slow the backstroke motion, the less involved are the forearm muscles and tendons. Then, when the bicep muscles contract, the radius bone only travels in one drection and that is straight, with little to no pause at the transition.
No other muscles are activated allowing a smooth transition from triceps to biceps muscle contraction.