The hook kick can be very fast, I was KOed by a reverse hook kick to the head and I barely saw it coming. I thought the guy was retreating.
This is one of the most powerful full-contact kicks. (In TKD, I believe it is second only to the back-kick)
The power in the hook kick comes from your abdominals, lower back, glutes, etc, combined with circular movement and the constant acceleration of your heel. These are usually weaker muscle groups (especailly the lower back) which need to be strengthened in order to allow for greater control. The actual bending of your knee doesn't increase the power of the kick that much and can potentially get your kick caught on your opponent's shoulder which is a *very* bad thing. The key is to get the extension of the kick to exactly match up with the time of contact, so that your leg is perfectly straight when your heel impacts their jaw. After you get a hang of this, if you want to be flashy, try the 360 Reverse Hook Kick. It's fun to do and very good for training balance.
This is a very powerful kick if done right but is next to useless in a self defense scenario because it's only real target is the head, which poses too much threat of your leg being caught.
[This message has been edited by Leo_E_49 (edited 03-21-2005).]