This is a good way to warmup the wrists and get them ready for more intense activities (such as balancing on your hands).
Get on all fours and place your fingers around the knees, with the palms facing up. Placing the fingers around the knees will help prevent rotation of the hands when performing the exercise.
Turn your elbows in and out, thus alternating between internal and external rotation at the shoulder.
Each cycle (turning in and out) counts as one repetition.
Form tips
It might help to slightly bend the elbows to initially learn how to perform the movement. Once you got it, straighten them again.