In the Taekwondo rear leg "off the line" roundhouse kick, the knee is first chambered before any body rotation. This differentiates it from the Muay Thai, karate, and other roundhouse kicks that tend to start the rotation before or during the rising of the knee. The knee is then rotated nearly parallel to the ground and the kicking hip is simultaneously rotated towards the opponent. The rotation of the hip, combined with the snapping of the leg forward, gives the kick its power.
In a front leg roundhouse, the leading leg is drawn up vertically, and rotated and snapped toward the opponent in the same manner as in the rear leg kick. A fast kick version is done by skipping forward with the rear leg, moving the kicker toward the opponent while simultaneously chambering and firing the front leg roundhouse. This version was effectively used by Bill Wallace, a highly successful Korean martial art stylist, during his full-contact karate career. The front leg kick is weaker than the rear leg kick because the hip does not rotate as much; however, it is faster because the leg travels a shorter distance before striking the opponent.
Aero Step Round Kick
Chamber rear leg as if performing a front kick, but only lift it a few inches. As the knee passes the support knee, retract the hip and simultaneously execute a round kick with the support leg before the raised foot touches the ground.
A difficult variation of an aero step with a round kick is to lift the front leg first and kick with the rear foot. This is a popular technique for counterattacking without stepping backward. The aero step is an effective way of adding speed and power to the round kick, because of the reaction force of one leg pulling back and the other kicks forward.