Enzuigiri

Literally a kick to the back of the head, though kicks thrown in the same method are usually called the same.

Enziguris started as a counter to a counter - the attack would throw a kick, the defender would catch the leg to stop the kick, and the attacker would spin around to kick with the other leg to the back of the defenders head. Over time, attackers have started to plant one leg on the defender to boost up into position for the head kick.