I don't pretend to understand what I don't understand either
Hey, Lefty penholder here!
The thumb, index finger, and the middle finger play huge factors in the penhold stroke. The ring finger sort of merges with the middle finger. For this post, consider the middle finger to be both the middle finger and the ring finger.
The fingers have to be firm; otherwise, the paddle will slip when you do basic multiball.
The further your thumb is down the paddle, the more firm. The more your middle finger touches the rubber, it will also be more firm. Also, wrapping your index finger might help here.
After a while, your middle finger might get deformed. My middle finger is slightly bent sideways away from my thumb.
Wrapping your index finger around will make RPB a little easier. The more wrist you intend to use, the more your index finger will have to wrap. If your index finger wasn't there, your paddle would probably fly away. how tightly you grip with your index finger will affect how sore your finger will get when you practice rpb; remember, it is the only finger preventing the paddle from flying off.
The thumb and middle finger are necessary to close the paddle. Also, the position that the thumb and the middle finger take will also determine what kind of angles you can make with your paddle. For example, penhold looks like shake hand, except that the paddle got rotated about 90 degrees. The position of the thumb and middle finger determine that angle. Ideally, that angle isn't too small or too big.
Having straight fingers on the back of the paddle makes it hard to efficiently use the wrist. What do I mean by this? Every swing requires there to be a backswing. Having straight fingers for the backswing on the RPB feels really weird to me. (I feel no power). For the backhand, you're fingers are supposed to be dragging or pulling the paddle forward. If your middle finger is bent, it is easy to pull. If you're fingers are straight, you're middle and ring fingers are now pushing the paddle forward. Also, curved fingers also act as a spring board to get your paddle back to a ready position. Straight fingers don't do that for you.
Also straight fingers force your paddle to be pointed a little more away from your body and a little more up. That may or may not be desirable. It's also the difference between the forehands of Wang Hao and Xu Xin.
It's okay for your thumb to push though
(pushing is the only thing the thumb can do here anyways).
All in all, your fingers are there for control, not power. You use your wrist, forearm, and body for power. You fingers manipulate your paddle's angles and the limits of your paddles angles. Your wrist and paddle should be able to freely move up and down, forward and back easily.
A bad penhold grip would be when the last three fingers aren't together. You need them to be together if you want your paddle to have a proper collision with the ball, especially if the ball is very fast and is going to the forehand. It also limits the freedom of the wrist.
The three fingers should be somewhere close to the middle of the paddle.
If the three fingers are too close to the thumb, then you can't close your paddle properly. If the three fingers are too close to your index finger, you can't open your paddle properly.
If the paddle is always pointing towards the ground for the forehand swing, it means that the wrist isn't being used properly.