Put your thumb and middle finger together at the tip. Press down, against each other. Slide them off each other, and the resulting slingshot effect will rocket your middle finger into your hand, causing the sound. Doesn't work every time, but I can hear a sound at least.
All that does is cause EXTREME pain to my ring finger, with the middle finger being nowhere near the palm (either in a small open space or touching the middle part of my thumb)