Nice guide. I really saw some parallels with piano playing techniques. I remember taking an intro class, and there is definitely a study of the tendons of the hand and how mechanics of your bones etc. function most efficiently for striking the piano keys. When I was reading that guide, seeing how the palm grip differed from the claw/fingertip, it really made sense how there is that parallel with efficiency of mechanical movement of the fingers.
For piano, you imagine like you are cupping a ball or orange and that translates to good playing. I think that's similar to the claw and fingertip grips on the mouse, so I will focus on those types of grips.
Odd to hear that guide talk about a pit of the hand. Not sure exactly what that is, but I think they mean the heel of the palm or inside palm somewhere that forms a pit when you cup your hand.