Originally Posted by tricky
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So, at this point, Sampras's racquet face has already started to open up because he's already started his lateral arm takeback. In your case, I think you were rotating your forearm to open up the racquet face, which you don't want to do. But, otherwise, yes you have the right idea.
One thing you want to notice is how far back Sampras's shoulder is from the first clip. This is a key thing I want to point out, because in your frame, the shoulder is no longer going back and now you're pivoting the elbow on its own accord.
At this point, you may ask -- well, my shoulder can't go back further, how am I supposed to pivot then? The body. The body drives the pivot.
Sampras's body orients itself much higher than yours (front side straightens and stretches) and it bends around more. This enables the arm takeback to continue, except that now the elbow will pivot. In fact, the elbow actually "sinks" a little into the trophy position if you coil around properly enough.
Plus, as you coil your body upwards, this also enables the higher ball toss and the deeper knee bend.
I also want to emphasize that the front/left rib remains slightly bent, and the back/right ribs remain straight through the windup, takeback, and initiation of the forward swing. This helps preserve the "balance" between the shoulders and the shoulder rotation that you want.
With both Picture 2 and 3, because the shoulder has mostly stopped taking back, what you have now is a concentric/positive movement or a "lat raise." The elbow pivots upwards; the wrist follows the elbow, the hand finishes raising it. At this point, pronation will not really happen even if you throw the ball way over to the left because the shoulder rotation is incomplete.
So the thing you want to nail down is that the takeback of the arm is continous, that the takeback is primarily lateral, and that whatever goes on with the elbow is coordinated with this lateral motion of the takeback. The elbow is not independent, and the elbow does not do anything without consent of the body or the shoulder.