Video courtesy of kyryllo
Use the kick if the opponent is backing up or is very defensive, also to end a combination. Don't use it if your opponent is known for rushing, shortening the distance or direct countering.
1. Start with a fake front or roundhouse kick (if trying the roundhouse make sure you don't throw the hips in becouse you won't have enough room for the kick later). The fake kick should be fast enough to cause a defensive reaction.
2. The next three things are important and MUST be performed simultaneously: 1)Snap the lower leg back and continue raising the knee; 2)Turn the hips; 3)Turn the supporting foot so that the heel shows to the target. Once again all these things SIMULTANEOUSLY!
3. If the lower leg and the thigh reach the same level begin to straighten your leg as fast as possible, the kicking action is performed in a somewhat down ward motion, achieved by turning your hips further while performing the last phase of the kick.
This is only a variation and not the only way to perform the kick. It's not an easy kick but very practical.