Almost every martial art I've been in over the last 30 years included some amount of board breaking. It can make for an interesting public demonstration of striking skill against a fixed target, but for us its primary purpose is the development of confidence. When someone sees that he has developed the precision, speed, firmness and power to break a board (or several), it can be a psychological boost, much like the pride someone might feel if he hits a bullseye when target shooting.
Board breaking is done in a number of ways. The board might be held by a person (at risk to wrists and fingers), in a rigid holder, resting on bricks, dangled from the hand. If the holder is massive and rigid, the size of the person becomes a big factor; large, powerful people can break many more boards at once than smaller, lighter people if the boards are held firmly in place. The less mass and rigidity there is to the board holder, the more important speed and accuracy become, somewhat negating the advantage of size.
There are a few different ways to look at this from a physics perspective. A transfer of energy, or an application of force. From the energy perspective, a typical estimate is that about 5 Joules are necessary to break an average pine board. 5J is very little energy. Someone who has never punched anything in his life can be coached to deliver many times this much power in a single session. Being highly unskilled, let us assume that only the mass of his hand and forearm is applied to the strike.
If it is true that it takes only 5J of energy to break an average board, and an average hand and forearm weigh ~ 1.3kg, assuming an inelastic collision, the necessary velocity of the hand is easy to determine:
E = 0.5 x mV2
5J = 0.5 x 1.3kg x V2
V2 = 5 / (0.5 x 1.3) = 7.69
V = sqrt (7.69) = 2.8 m/s
So our striker needs to hit accurately with a velocity of about 3 meters per second, with a tight fist. Studies of skilled boxers and martial artists indicates striking speeds between 10 and 15 m/s. The assumption that an unskilled person might have 1/3rd the speed isn't outrageous.
Insofar as the puncher can incorporate his body's mass into the punch, his applied power increases dramatically. With a skilled striker, it isn't a matter of getting hit by a 1.3kg fist at 10m/s -- he didn't cut off his arm and throw it at you. Assuming he can get only half of his body's mass into the strike, even at the expense of some velocity, now you have a 40kg fist hitting at 8m/s, for a total available energy of:
E = 0.5 x mV2
E = 0.5 x 40 x 82 = 20 x 64 = ~1300J (!)
The striking energy does "work" on the target; deformation and/or displacement. If the board is held rigidly, then it cannot displace and all energy is applied toward deformation, resulting in compression along the face of the board and tension along the back face. When the tension exceeds the board's integrity, it ruptures, and everyone is impressed.
Things get really complicated when the board holder is not rigid. Now the board can displace and, insofar as it does, energy that would otherwise cause deformation gets transferred through the board into the holder, making it more difficult to break the board. The less rigid the holder, the more critical are speed, accuracy and the hardness of the striking object.
In Via Potentia we have normally factored in weight by requiring people who weigh more to break more boards to pass the exam (one additional board per 100 pounds of weight). This made sense for massive or very rigid board holders like we were accustomed to using. In such a setup, the boards take the full energy of the blow. However, we recently acquired a board holder that mounts on a Century Wavemaster base. The result is that the holder has a substantial amount of flexibility, and the whole thing can be pushed. When the board holder moves, tension within the board is released. This changes the game entirely to the point where a person's mass is almost a liability, since larger people tend to be slower (though more powerful).
The big guys in the program aren't thrilled with this new holder because they can't break as many board with it. But in some ways this is a desired (if unanticipated) result. Our desire is to be able to strike fast, hard and accurately enough to cause an injury. Striking a completely rigid target, though, does not similate that. Rather, striking a target that displaces when hit comes closer. In this way, board breaking using a flexible, movable holder may be more useful for training than using a completely rigid base.