An in the footwork looks very similar to Ji. However, the energy (as you know from Grasp the Bird's Tail) is quite different. An is like a long wave, beginning in the rear foot and pushing towards the front. However, lacking the upper body and waist action, using An in the legs has less power than the actual "Push."
The parts of the legs which manifest An are the knee and the thigh (remember, we're talking about steps, not kicks). Using the knee is very similar to Ji, except that the ankle is not locked, and the power comes from pushing with the back leg rather that pulling with the front leg. This may, therefore, be used against the knee, or the inner or outer thigh, as the situation and positioning dictate.
Using the thigh requires initial contact in close range. As you step in, make contact with the opponent's leg. Pushing from the back leg into a forward stance, you can force the opponent off balance with the pressure of your front thigh. This is especially function when combined with the hand form. You may, however, need to experiment with different angles to really get the feel of An moving from back to front.
SB