What is a 540 kick?

Answer 1

It is a jump-spinning kick (popular as an advanced kick in Taekwondo), where the rear foot of a fighting stance is used to hook kick the target after having twisted backward 180 degrees lifting the rear leg like a twist kick, jump and continue to spin another 180 degrees as though throwing a tornado roundhouse, and finish with the final 180 spin hook - all while in the air. Often times the kicking foot starts in the front leg position, and a lead in step is added to build up momentum, but counting the 540 rotation does not accurately begin until the rear (kicking) leg is lifted off the floor, even though the front leg is still on the ground at that point.

(see related link of Korean Tiger Taekwondo team member demonstrating a board break with the 540 kick).