Newton’s Third Law of Motion states that for every action there is an equal and opposite reaction. When a cannon charge explodes, the cannon barrel concentrates the force of the explosion and uses it to force the cannon ball out the end of the barrel with great force. However, an equal amount of force is exerted backward against the cannon itself in reaction to the force applied to the cannon ball. If the cannon is firmly attached to the ground, this reaction force is reapplied to the cannon ball to propel it with even greater force. However, if the cannon is on wheels and is free to move, the reaction force will cause it to roll backward and the cannon ball will fall far short of its intended target.
When a punch has an action force applied to it, an equal and opposite reaction force is applied through the body down to the base and then to the ground. If the base is firm and the body is stable, this reaction rebounds off the hard ground and increases the force of the punch. If the base slips or the body is unstable and causes the reaction force not to be applied directly to the base, the force of the punch will be diminished.
Another type of action-reaction occurs within the body. If you stand in front of a person with both fists out in front of you with the arms parallel to the ground and someone pushes straight back against one fist, the body will rotate at the waist (its fulcrum point) and the other fist will be pushed forward. The same principle (called a couple) occurs when one applies a force down on one end of a seesaw. The seesaw rotates at its fulcrum point and pushes the other end up. When punching outward with one fist, if you pull the other fist quickly backward, the reaction force from the retraction will increase the power of the punching fist.