I recently thought of an analogy to describe how to execute a double kick on a single pedal. The skater going up the half pipe represents the leg lifting up using the thigh muscles first. But just before it gets to the top you press down with your toe using the calf muscle. This is like the skater kicking off the ramp just before the top. In the air and after the initial kick the leg and the skater are completely relaxed as they let the momentum bring them the rest of the way up and then let gravity take them down. Just before the leg comes all the way down the toe pushes again to make the second kick. Likewise the skater must land before coming all the way down. This type of double kick allows the muscles to relax and keeps the drummer from tiring out. Practice this technique keeping in mind that the most important focus of the exercise is staying as relaxed as possible. The muscles should be completely relaxed except for the instant the thigh moves the leg up and the calf presses the toe down against the pedal. The muscles should turn on and off as quickly as possible. It will be 3 quick "on-off's". The thigh first to begin lift off of the leg, the calf pressing the toe down, and then the calf again pressing the toe down as the leg falls back down under gravity. Finally, the leg and foot should be at rest and completely relaxed on the pedal until it is needed again.