The Cal Ripken approach....try swinging a tennis racket with only the bottom hand...same bottom hand/wrist action....same as golf....we have players backhand a pad to get the feeling as well instead of karate chopping....forearm needs to supinate...
As most know I'm new to really understanding the swing. So if I demo the swing then, yes I was palm up,palm down at contact. I know this was/is wrong,but demo it like that. I don't know why maybe that is what I thought was correct. So out of this discussion and the gif examples I see what the proper hand position should be and gif of what it looks like. Also videos of different ways to possibly reinforce the movements to properly teach and learn. Another day of learning something new.
Grab the bat and do a top hand only swing. Fingers 90 degrees to the bat.
Now grab the bat and do a bottom hand only, holding it angled in you finger tips. From tip of pointer finger to base of pinky. Now do a back hand tennis swing.
Now do both hands. I like and teach the team girls a split grip, just like an inch gap between hands. Now swing, whipping top hand past bottom hand.
I don't think "palm up palm down" or close to it is necessarily wrong, just a different road to the same destination.