How do wood boards break?
If trees were rigid, unmoving objects, they would snap in high winds. However, the composition of wood allows trees to flex with the wind, absorbing the force of the wind without breaking, at least until their flex limit is exceeded, and then the trees break. When a single board is struck, it bends like a bow, until the flex limit is exceeded and the internal fibers in the board begin to tear apart. When a board is flexed, the bottom surface must stretch more than the upper because it moves in a larger arc of a circle. This means that the bottom will start to crack first. Once cracked, the wood becomes very weak along the break along the inside of the crack and the crack immediately starts to deepen into the wood. The board actually tears open form the bottom to the top.
When more than one board is used, the bottom board feels more stress than the other boards since there is no board beneath it to resist its flexing. When the bottom board breaks, the break propagates through each board from the bottom up, breaking each board in turn. Since the stress needed to break two boards is greater than the stress to break one board, when the bottom board breaks there is suddenly a much larger stress on the top board than is necessary to break it, so it breaks quickly.
If the strike is too weak to break the boards, they will only bend, storing all the energy in their fibers. Then the boards will transfer that energy back into your hand or foot, and pass it along through the joints, causing pain or injury. Make sure to use enough arm or leg motion to force the boards past their maximum flexing point or all your punching energy must be absorbed by your body.