An alternative way of thinking about the strike is in terms of energy. Energy is a measure of how much work something can do. Work is done when a force causes an object to move. One form of energy is kinetic energy: the energy something has due to its motion. The equation for the kinetic energy of my hand is:
Where m stands for the mass of my hand, and v stands for it's speed.
Energy is always conserved. This means that the kinetic energy of my hand just before the impact is equal to the sum of the energy used up in breaking the block (called the deformation energy) and the kinetic energy of the block fragments and my hand after the collision. Some of the energy is also lost as heat energy (the wood and my hand heat up slightly), and sound (the sound of the wood as it cracks).
The more kinetic energy my hand has, the more energy I'll be able to put into breaking the block. Because kinetic energy is proportional to the square of the speed, it makes sense for me to maximise my hand's speed.