In the Grips Fundamentals section of the website, we talked about how your grip affects your contact point. Because your grip affects your contact point, it also affects the amount of upper body rotation you will naturally need to get. The more extreme your grip, the more you will need to rotate when you make contact and when you follow through.
Take a look at the video above at 0:20. If you have an eastern grip and you swing to your contact point, you will make contact when your body is rotated about halfway towards the net, maybe a little more. If you change to a western grip at this point though, your racket will not be in the correct position to make contact. You need to rotate your upper body further around to get the tennis racket’s strings facing the correct direction (and you also need to raise the racket up slightly).
Again to reiterate, you need to turn your body progressively more towards the net to make contact with the tennis ball as your grip becomes more and more extreme (more towards the western grip). Because you have to rotate more to get to the proper contact point with an extreme grip, this also means that you need to continue to rotate around more on your follow through.
At 1:10 in the video above, we take a look at a couple different tennis pros making contact with different grips. In the first picture, Radek Stepanek is making contact on a service return using a continental grip, which is very conservative. His upper body is rotated only about halfway towards the net. This is because you barely need to rotate your upper body around to get to the correct contact point (with the strings facing the correct direction) when using a continental grip.
The next picture in the video above at 1:50 is of Marat Safin’s forehand. You can see that in order to get his strings facing the net, he has his upper body basically facing the net. You can see from these two examples that the amount of rotation you get in your upper body is very closely related to your choice of grips.