A Single Tap
To play a single tap, I let my hand fall down and to the side, pivoting mostly from the elbow. Some people move their hand more straight to the side. When the hand reaches the end of the arc, it stops suddenly, the stationary bone stops too, but the moving bone which is held more loosely keeps going and collides with the stationary bone. The fingers don't move, they just hold the bones in the correct place with the correct tension, the movement of the hand is what makes the bones "dance." The wrist should stay loose so the hand naturally rotates when it reaches the bottom of the arc, and "bounces" back on the way up. The wrist angle stays the same throughout the movement -- the back of the hand is bent a bit towards the back of the arm. See the page on the grip for more help.
Most people when they start out try to "make it happen." They focus on making the ends of the bones hit, and either use their fingers to push them together, or try to push the bottoms of the bones upwards towards each other. Try to resist this impulse! It might help to think of pointing the tops of the bones (the part sticking out of the top of your hand) suddenly away from you. Think of your arm as the spokes on a wheel or the hands on a clock, the elbow at the center and the hand traveling around the circumference. It may take some perseverance to make it happen, compare your movements to the animations here and keep trying.

Don't Do This!