Simply put, an immobile shoulder provides a quality hub for the shooting arm and pool shooting hand to wheel about in a pendulum-like motion.
I say that the lower arm takes a pendulum-like motion as the many possible flex motions and directions of the wrist and fingers provide for something less than a true pendulum. The movement is more like a pendulum with a flattened base allowing for a lengthy time of straight motion as pool stick meets ball.
Do I hold the shoulder rigidly in place? Absolutely not, any untoward rigidity or muscle clenching is poor pool and leads to fatigue at the table. But I follow the steps I outline for a great stance each time I step up to play. My shoulder is already in a good place and from there, needs no adjustment during most any pool shot (at least until contact has been made with the cue ball).
Some players move their shoulder much during the final stroke. Start with adjusting your stance as I recommend, and be aware of your shoulder's movement during the stroke. Simple awareness will often help correct the problem instantly, with an instant improvement in play, also!
Do return to this thought at times in your career and keep checking on that shoulder. You'll be glad you did!
