In the second and third articles in this PDF http://www.sfbilliards.com/articles/2004.pdf
are some thoughts and observations on what the elbow does during the stroke.
In my experience, players often have two distinct strokes. On soft shots their upper arms are motionless and the stroke is restricted to the forearm. The shoulder is frozen in place and moves not at all. On power shots, the elbow drops at the end of the stroke. In my view, this drop is essential on power shots to keep the arm happy and not hurt. I've tried a still elbow on such shots and it hurts. The drop is typically the thickness of the upper arm, so a few inches.
This observation applies to players as diverse as Allison Fisher and Nick Varner. Well, they're not that diverse since they are both Hall-of-Famers.
In the case of snooker players, keeping the chin on the shaft forces a piston stroke, mostly. That will naturally lead to some elbow drop on power shots from the simple mechanics involved.