The start of the Back Hook Kick is similar to the start of a Back Kick: in both cases you normally start-off by facing the target, then pivot away from the target, and while facing away from the target, you kick. The significant difference is that the Back Hook Kick is an arcing kick while the Back Kick is a linear kick. In other words, for the Back Kick, your leg is thrust straight backwards like a mule, whereas for the Back Hook Kick your foot comes around to the target in a wide arc.