Yesterday I completed my first martial arts board break, as practice for my upcoming brown belt test. A few weeks ago, I bought a one-inch-thick, one-foot-wide piece of pine shelving board from the hardware store and cut it into one-foot-square boards. I broke a stack of three of these boards yesterday, with a side thrust kick.
By chance, there was a new student in class yesterday, to see what it was about. He saw the stack of boards before class, poked at them a bit, and asked what they were for. Sensei told him that I would be breaking them. He seemed interested, and started asking questions about the process.
Most people are not impressed by board breaking in demonstrations, because they assume it is some kind of trick or it uses flimsy boards. But this guy had a chance to inspect the boards beforehand, so he could see that it was no trick. He saw me send my heel through three inches of no-nonsense hardware-store wood.
Actually, there is a bit of a trick. You are splitting the board along the grain of the wood, the same way you do when you are chopping up cut logs for firewood. You have to make sure that the side of your foot hits the boards parallel to the grain, so you are attacking the weak point and the board will break cleanly. The three boards have to have the grain lined up the same way. Nobody could break through three inches of wood if the grains were crossed like they are in plywood.
You also have to make sure that the board is dry, and that the people holding it know what they are doing. But even so, it is not an easy thing to do. Like most things in martial arts, it takes speed, fluidity, focus, and accuracy. It is not a matter of brute 'hulk smash' force. The key is to make sure your foot is moving faster than the board can bend and absorb the impact, and that you sustain that speed through all three boards.
It is a true test of ability, one of the few ways that you can safely do an objective test of your training. That's why we do it. Every other testing standard is either the subjective evaluation of the tester, or your performance in a controlled sparring situation. This is something real, testing your skills against a fact of nature.