Also called a donkey kick, spin kick, mule kick, or turning back kick. This kick is directed backward keeping the kicking leg close to the standing leg and using the heel as a striking surface. Most often, this kick is delivered with a spinning motion in tournaments. It can be highly damaging due to its power and Benny Urquidez stated once in his book, “Training and Fighting Skills”, his belief in it being the most powerful kick in karate.
[From Wikipedia. Original page is here]