I know the extended arm has to with the centripetal force necessary for the spine roll-which-rolls-the-shoulder-which-rolls-the-hand, but I don’t know how to say it, and I’m sure other elementary principles of physics can be educed.
Pitcher’s crouches and bowler’s approaches just before release come to kind.
ETA: Not for the first time, allow me to thank SD for giving me the opportunity to use a fancy word I don’t think I’ve ever said or written. What’s weird is I didn’t set out to do it, it just got typed.
I played a little QB. The non-throwing arm and hand starts the rotation of the shoulders and back and adds to the angular momentum of the upper body so that the throwing arm rotates around faster and imparts more tangential velocity on the ball - much like cracking a whip.