Originally Posted by sureshs
Very very interesting. Is there a pronounced going over the ball at contact in the dwell time, or is it just recoil?
when you push against an object at a point away from the center of mass it will rotate.
and since the ball has inertia that rests against a change of direction the upward moving racket face will brush up the ball causing the ball being below center at some point. this causes the racket to twist and "go over the ball".
the ball creates a hinge for the racketface to rotate around.
the tennisspeed guy himself proves that the racket is not turned by the hand. at about 3:00 he shows a shot of federer with his racket face opening at contact on a above center hit
of course he believes that pros deliberately hit below center so that the ball can turn the racket over...