Some interesting sites to check out -

"A detailed description of the physics of karate has appeared in a Scientific American article. 1 We will only present the salient features here relevant to the impulse momentum theorem. First we estimate the momentum of the fist. Its effective mass is greater than just the hand ... after all, you're swinging the whole arm to which your fist is attached. Resting the forearm on a scale suggests an effective mass somewhere between 2 and 4 kg. The viewing of a videotape showed the hand swinging through 1/2 meter in the time between frames (1/30 sec) thus establishing a velocity 2 of 15 m/sec and a momentum of 45 kg m/s.

Wood is a fairly elastic material and a 30 cm long board typically requires about a centimeter of deflection before breaking; 500 Newtons will deflect it to that extent. If we assume that a 2 cm thick board actually stops the swing of the fist, the action takes place over a distance of 2 cm + 1 cm = 3 cm and the time of this interaction will be the distance divided by the average velocity of the fist = (vi + vf)/2 = 7.5 m/sec. Thus t = 0.03 m 7.5 m/sec = 4 millisec. In this time the momentum of the fist would change by 45 kg m/s. That's equivalent to 2250 lbs! Of course the board breaks with this kind of force and does not stop the fist.

The boards are stacked on top of one another and kept slightly separate with pencils placed between them; rupture can proceed successively through the boards with each rupture involving a smaller force than if a single thick board were used. This also has the effect of the momentum of the downward-moving broken pieces of the top board helping to break the board beneath it, and so on down the stack. Thus the peak force to break, say, eight boards is less than eight times the force needed to break one board."
"In case you ever wondered what it's like to be knocked out, it's like waking up from a nightmare only to discover it wasn't a dream." -Forrest Griffin