The easiest test that everybody can is just grab your racquet and extend your arm fully next to your head. Extend it completely as high up as you can and then try to pronate with your forearm. Pronation is the part from the elbow to the hand where we open up the palm of our hand. You see that the impact on the racket head is actually not so big. You see it turn and this is when the pronation is only giving direction to the ball. We go a little bit more out wide; we go a little bit more slice. That’s what the pronation is doing when we have our arms completely stretched.
The same goes if we now go to the upper arm between the elbow and the shoulder, we can internally rotate that as well. You’ll see when you do this with a complete straight arm the impact on the racket head is not so big. It helps the pronation a little bit, but in terms of the racket head speed not much is happening.
Now try to do this same movements with your arm slightly bent, not 90 degrees. It’s a little bit less so if there’s a small bend in your elbow and then do the same movement. We start with the external rotation when the racket going back and the internal rotation. You see what happens with your racket head. All of a sudden, a big range of motion happening with the racket head. So that is why when we are talking about this multi segment serve, starting from the bottom up with our leg drive, hips, shoulders and ending with our arm. Our arm should not be completely extended because we lose too much racquet head speed if we try to hit the ball as high as we can. Clearly, the higher we can hit the ball, the better. But it should not be with the compromise of extending our arm completely because we still need to be able to have a small bend in the arm and a small angle in the wrist as well. So, we can really use this internal rotation to smack that serve.