Palm-up/palm-down is the result. Hitters hands are working from radial/ulnar deviation when whipping the barrel "down to" the ball.
I think top hand wrist extension is in there as well. Keeps the barrel up or through the ball. Hence Palm up. Take a look at Pujols ‘feel’. Too much radial is bad for a swing imo. Creates wrap and wrist angles you can’t quite come back from. More is not always better. I would say 90% of elites have that top hand wrist extended earlier than later.
Edit: wrist extension kills some radial deviation. Pujols is w prime example:
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