NEWTON'S THIRD LAW
For every action, there is an equal and opposite reaction.
The statement means that in every interaction, there is a pair of forces
acting on the two interacting objects. The size of the forces on the first
object equals the size of the force on the second object.
The opposite hand pulls back to the belt, as the other hand punches. More specifically, when punching, the hand moving forward exerts a backwards force on the body of the person performing the punch. Taekwon-Do teaches us to counteract this force by puling the other hand backwards at the same time, causing a balancing forwards-force on the body. This greatly increases the power of the punch.
When a hammer strikes a nail, the nail exerts an equal and opposite force back on the hammer.
A rocket taking off from earth which pushes fuel in one direction and the rocket in the other.
A swimmer pushes backward on the water and the water pushes back on the swimmer, propelling her forward.