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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.
take a look at the shoulder and you'll see that its not really loaded, whatever loading was taking place is all screwed up, maybe due to the down wrist position.
instead of the shoulder muscle being parrallel to the back fence, it is then perpendicular....
At this point, you're still generating terrific power from your body (and you still get a upward swing), which is why you do hit the ball hard. But the arm doesn't whip and you don't feel like you're hammering down on the ball (i.e. pronation.) It again goes back to the same thing. The arm is primarily interesting in taking back the shoulder and making sure the front-back shoulder balance stays (which is helped by the elbow.) It's really the rotation and twisting of the body that will set up the trophy position.
In a way, the pronation itself is a reward
for the rotation being correct. You would feel as if your pinkie easily slams the forearm down onto the ball, and that the overall motion is like a hammer coming down onto the ball.
Or to put it another scenario:
Say you don't twist your body upwards. at all You should expect your racquet arm to be no higher than sternum level before you initiate the forward portion (i.e. turn hips forwards) of the stroke. Even though the swing is wrong and will lead to the ball traveling forward instead of down, the rotation of your shoulder is correct. The elbow will still pivot around so that you will have a racquet drop. The actual motion of your shoulder is still correct, and you'll still experience pronation.
However, say you now try to simulate a trophy position by pivoting the elbow upwards at the end of your takeback before you initiate the forward swing. The hand is high at head level. The racquet drop decreases. The pronation significantly decreases. The rotation is off and starts to feel slower.
What this says is that the body dictates the trophy position and the pivoting of the elbow. The trophy position does not exist without the upward orientation of the body. The body enables it; the body drives it.