Originally Posted by Chas Tennis
Two things that could be improved -
1) For those who would imitate his body position, in my opinion, he keeps his shoulders too level. That orientation differs from the hitting shoulder high by lateral trunk flexion as recommended in the Ellenbecker shoulder video on minimizing impingement risk.
2) He also - as in nearly all serve instruction videos - does not mention by name or identify internal shoulder rotation and the prominent part ISR plays in developing racket head speed on the serve. Some instructors call the motion pronation but still do not clearly describe it.
You make some great points.
I also thought that his keeping the shoulders level might be counterproductive. For the sake of solely developing a feel for the racquet drop, though, it is a very good drill.
As for ISR: I just tried shadowing a serve without using ISR (but keeping pronation) and it is extremely awkward. What I did notice, though, is that pronation enables ISR; one motion facilitates and guides the other. I think coaches focus on pronation for two reasons: control (since forearm is closer to the ball that the shoulder) and so as not to confuse a student with two separate movements.