Point of focus
Technique: Chris's advice is to pretend that the target is beyond where it actually is.
Explanation: This is related to the need for speed. Beginners naturally slow down as they reach the target, to avoid hurting their hand. If I do this I could end up hurting my hand even more, as I might not have enough force to break the wood. All my downwards momentum would be absorbed by the wood as it bends slightly, and then directed back at my hand as the wood springs back to its original position. Ouch!
Use of body mass
Technique: Chris puts more of his body mass into the strike by snapping with his hips and moving his whole body downwards as he strikes.
Explanation: Kinetic energy is proportional to mass, so the more mass I can put into my strike, the greater my force will be.
Technique: Chris turns his hand upwards just before the point of impact.
Explanation: I think that one of the advantages of this is that it's best to hit the wood with the smallest possible surface area of my hand. This will maximise the amount of energy per unit surface area that I transfer to the board. I don't need to put energy into the whole board, just enough energy into a small area, so that the wood fibres in that part of the board shatter.
What I've written so far are just theories about how the block breaking works. To confirm what's really going on I need to do some experiments. The most important thing to test, for the sake of my own safety, is whether it's possible for me to break these boards. To test this I'm going to measure the speed of my arm as I practice the strike. I can then work out if my strike generates enough energy to break the board.
For the amount of energy needed to break the wood I'm using a value from a physics paper (Wilk, McNair, Feld, 'The Physics of Karate', American Journal of Physics 51, 783-790 (1983)). The value is 5.3 ± 2.8 J (J stands for Joules, a unit of energy). The ± 2.8 J bit means that the value is accurate within 2.8 Joules, so the energy needed to break the wood could be as much as 8.1 J. I've made sure that the piece of wood I'm breaking is the same type and size as the wood that the measurements were taken from. Now I need to work out how much energy I can put into the wood with my strike…