Fun fact: taekwondo students are taught to retract kicks faster than they are thrown.
Fun facts: I once saw a black belt in taekwondo get knocked out with the first punch in a street fight. And early in the MMA era, the world Taekwondo champion got beat into hamburger meat by a used car salesman with no special training. It was painful to watch.
Taekwondo is a sport art and while it teaches balance and quickness, it teaches you nothing about punching power or how to get hit. In fact, the techniques it teaches are designed to not hurt the sparring partner. Unfortunately, not everyone in the sport is taught that.
Pulling your kick back quickly prevents you from throwing yourself off-balance.
Sure, but it also prevents you from imparting significant momentum to your opponent. Imparted momentum is equal to force times mass times the moment of inertia which depends most critically on the time the objects are in contact. The faster you retract, the less you push into and through the thing you are hitting and the less damage you do. Compare with an actual combat technique like gastrizein:
[ame=http://www.youtube.com/watch?v=aPvJHIyFTII]Human Weapon Pankration - Gastrizein, Straight Heel Kick - YouTube[/ame]
That kick alone was recently used to put a guy not expecting it out of a fight in MMA. The greeks taught it for shattering an opponent's wooden shield. It's battle tested and it doesn't depend on magical thinking.
While it is perfectly possible to stun someone with a punch or a kick, I agree with Celebrim that anything my character can only do X times per day is in some way magical. (I don't much care about the fluff, but "X/day" as a mundane ability strains my belief too far.)
The basic problem with 'Stunning Fist' is that if it is merely hitting your opponent hard, why is it not tied to damage? Wouldn't that mechanic be basically a 'power attack' - targeting a soft point with a hard attack - as sort of a called shot? Why would it not be an at will ability? 'Stunning Fist' is limited to a number of times per day because its tied to channeling your 'chi', and presumably you only have a pool of available chi of limited size.
The truth of the matter is that most ancient eastern martial arts have been tried and found wanting lately. Oddly, it's the ones that got less attention back when kids believed ninjas really could jump over school buses - Filipino, Indonesian, Thai - that turned out to have the most practical ideas.