"Ideally, you want your armswing to stop once your shoulder turn has been completed, so your armswing and your trunk are in-sync"
Is there any validity to this statement?
I think that if your arms keep going after your shoulders stop in a conventional backswing, the only place the your arms can go is straight up, and for most folks, that puts them in a position to come over the top, or to try and manipulate the arms to achieve an on plane down swing.
Tank you can prevent too much arm lift (if you insist..) by not allowing the left scapula to raise. Let it slide to the left and right, but not upwards..
So when you get to the point that your body stops rotating you allow the left scapula to pull away from your backbone and your right scapula to slide towards it, but not raise towards your head...This give you the extra 20* for the full 90* turn
I fully agree. That is an excellent thought for completing the shoulder turn.
What I have a problem with, is arm travel beyond a completed shoulder turn.