That's a very good point... I've been poor at placing my finger sideways I think. I do it sometimes, then I forget, and revert back to old habits.
Gives more control on RPB than when it lies flat... Also makes the angle of RPB more "natural" to make. Those "minor details" I think are more majorly important than we think.
Though it also makes the fh feel a tad different, and probably not any more stable. Just a more BH oriented grip.
Am I right?
The reason why you want the side of your finger on the paddle is because of what happens when you use your wrist.
Try using your wrist with your fingers flat on the paddle. Then, try using your wrist with the side of your middle finger on the paddle.
EDIT:
Try doing backhand counter topspins (not drives) and flips.
There is a limit to what angles you can get when your fingers are flat.
The difference becomes very real and very important when you push and strive to get into high level play.
If you are just playing casually, or under 2000 USATT, either is probably fine.
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As for whether it affects forehand play, it hasn't really affected me in any way. Any player that has seen me play understands that my forehand gives me my best shots and is what I rely on.
It might be different for players who are switching/adjusting their grip. Either way, your middle finger is still important and is still able to do its job when you make your forehand shots.