For every action there is an equal and opposite reaction. Taekwondo makes use of this principle to develop powerful techniques.
One way reaction force is used is the way the pivot foot is used to thrust against the ground when shifting. The reaction force of this thrust is returned through the leg and hip to thrust the body forward.
Another way reaction force is used is through body rotation. Body rotation is created by anchoring one side of the body and using it as a pivot. The other side of the body is driven forward by the rear leg and hip as a reaction of the supporting leg's thrusting against the ground.
Hip snap may also generate a reaction force. In hip snap, the hip and leg motion, and the corresponding reaction force, is a short term, small scale pulse of power immediately followed by recoil in the opposite direction. Power is applied in a sharp pulse of energy of very short duration. Hip recoil also helps maintain stability.