The ap dolyo chuggi (or sometimes just dolyo chuggi) is probably the most used kick in Tae Kwon Do, at least in any sparring situation. Ap dolyo chuggi translates as "front round kick," which means it's the good old familiar round-house kick, except with the Taek Kwon Do twist that this node describes. The back-leg ap dolyo chuggi is made up of three steps, or movements.

1. Chambering the kick and turning: In initial practice, the first step is actually split into two steps, but evenutally the two steps become one fluid motion. The back leg is raised and bent, coming to its final position as close as possible to the knee of the planted leg. While this takes place, the planted leg should turn 90 degrees to the outside of its straight position. After these two motions, the body should still be facing forward, waist bent so as to retain as straight a standing position as possible. The knee of the kicking leg will be completely bent and almost horizontal. By the way, wherever your knee is pointing is where the kick will land.
2. The snap: As with all kicks, the snap is where the kick actually takes place, and the snap should ALWAYS come back faster than it went out. The leg should be snapped out as horizontally as possible. If the kick comes to the target diagonally, there won't be nearly as much power generated by the kick. The striking surface on the foot is the top of the foot, and the toes should be pointed.
3. Returning to rest: Returning to rest with the ap dolyo chuggi is basically the first step in reverse. After the kick, the foot MUST be returned to the chambering position, and then returned to rest.

This node is intended to only be a definition. Please seek out real martial arts training if you have none before attempting this kick. Believe me, without proper training, you are more of a danger to yourself than anyone else.