When you snap your fingers, what makes the sound? The friction of your two fingerpads or your finger hitting your palm?
I’m pretty sure it’s the finger hitting the palm. I just did a little experiment, by putting a piece of tissue paper where the finger hits the palm. This effectively muffled the snapping sound.
The finger hitting your palm. Put a piece of cloth or folded tissue paper there and observe the effect.
Is it the finger hitting the palm, or is it the compression of air in the pocket formed by the palm, ring finger, and rapidly descending middle finger?
If I hold a wadded-up tissue or cotton ball in that area (effectively damping any air effects), only allowing the tip of my middle finger to strike the palm, I can still make the snapping sound. Interestingly, the quality of the sound changes. It’s lower-pitched and less sharp. So I have to wonder if there isn’t some truth to your theory.
Try snapping with your ring and pinky fingers in the air (not cupping your palm).
I think it’s probably a combination of all three. The finger hitting the palm, and the air in the “ring-pinky pocket” being compressed seem to be pretty well supported by previous poster. Now, hold a piece of paper between your thumb and middle finger and snap. Make sure you hold it strongly with your other hand, so that it doesn’t end up hitting your palm and supporting the finger-against-palm theory all over again. It still sounds different, so I believe the friction between the middle finger and thumb is a third factor.