Its basically a punch where you use your arm as a whip. All good strikes start at the floor, in this case a rotation around your central axis travles from feet to hips to should and out the one hand.

The major difference is that rather than keep the joints in your arm tight and locked for the punch you keep them loose and your arm moves in a whip like fashion as the energy generated by your body travels up. If you do it in a "backhand" motion it looks a little like a class "bitch slap". The easiest thing to do is get a whip and watch how it moves...

Hope this makes some sense.

Guro Andreas