I am not saying that some wrist cupping is bad. He is just getting too much. I would suggest you look at some of the top PBA bowlers and see if any of them cup their wrist as much as he does? I looked at higher rev pros: Weber, Mallot, Duke, Barnes, Jones, and Rash. Rash is the only one that gets close and he is still only going slightly past a 90 degree thumb. I would suggest that you should always try to emulate the top PBA bowlers. If there is something that they all do, or don't do in their form I think that is a pretty good hint that we should all be looking at doing it that way.
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