According to Newton's Law, every force has an equal and opposite force. When an automobile crashes into a wall with the force of 2,000 pounds, the wall will return a force of 2,000 pounds; or forcing the end of a seesaw down with a ton of weight will provide an upward force of the same weight; if your opponent is rushing towards you at a high speed, by the slightest blow at his head, the force with which you strike his head would be that of his own onslaught plus that of your blow.

 

The two forces combined: his, which is large, and yours, which are small, are quite impressive. This then is the reaction force from the opponent. Another reaction force is your own. A punch with the right fist is added by pulling back the left fist to the hip.