The Spin kick! This is easily the most beautiful of the basic kicks, and the most powerful. It can be delivered from offensive or defensive stance, but only with the trailing leg. Delivered high, it strikes the opponents head, frequently resulting in a knock-out. Delivered at medium hight, it can break ribs. Delivered low, it is an incredibly effective sweep.
From offensive position, turn your body clockwise, and look over your right shoulder in the direction you were originally facing. That's right - you turn 360° with your torso,and you are at approximately 180° at this point. Your left leg should be bent at the knee (otherwise you'll fall over!), and as your torso swivels and you look over your right shoulder, your right leg will automatically start to follow in the direction you are looking. Keeping the right knee straight, whip your leg around and back to your starting position, in stance. The striking surface is the back of the heel. Your foot should be on its side, toes horizontal and pointing away from the target. If you allow your toes to point up (easily the most common mistake for beginners), you will a) strain both the knee and the hip of the kicking leg and b) move the striking surface from your heel to your ankle bone... ouch!
The spin kick generally describes a circle, but it should be noted that there is a plane of motion within which you can strike. As you spin around, and your kicking leg moves between around 160° and 200°, your foot should be on its side and there should be no change in the hight of the kick - describe a straight line with your foot accross these 40° of the turn, and then bring your foot back down and into stance quickly. If performed correctly, you can break 3 boards in a row, each separated by a few inches, as your heel traverses this plane.