Speed, Power, and Contact time
Factors that influence board breaking are: speed, power, and how long the hand or foot is in contact with the target. Collisions similar to a bouncing super ball are nearly elastic, since they conserve both kinetic energy and momentum. When punching a board, unless the hand has an apparent mass much larger than the board, it will be stopped on contact and the center of the board will begin to bend some from the impact. This type of impact will cause the greatest transfer of energy to the board. This type of strike, called a "speed break," is where a board is suspended by one side. This mean the board must break before it is knocked from the single supporting hand. This type of break requires much training and experience to generate the speed necessary to accomplish the break, so it is probably beyond the ability of a beginner.
A simpler break involves an inelastic collision. In this type of collision, the hand and board remain in contact through out the strike. As the object is struck, the hand continues to exert force and the center of the board begins to bend with the velocity it gains from the collision. The strike continues to apply force past the point where the board reaches its breaking point, accelerating through the board as it breaks. If the hand is moving too slowly, the flex of the board will stop it before it breaks. It would be like trying to break the board by leaning a heavy object on it. At greater speed, the hand will slow as it contacts the board, however, the continued application of force by the arm that is larger than the force the board exert, will keeps the hand's speed high. Eventually, the board will reach maximum flex and break. With high speed punches, the hand will always be moving faster than the board can react.
So do not try to hit the board with all your strength, try to hit with the greatest speed possible. You are not trying the "kill" the board, you are trying to pass though it.