The "reaction principle" is a bit broad. It could refer to several things depending on the context. Can you provide more information or context about what you're referring to?

For example, are you asking about:

* Newton's Third Law of Motion: This principle states that for every action, there is an equal and opposite reaction. This is the fundamental principle behind how rockets work, for example.

* Reaction mechanisms in chemistry: This refers to the step-by-step process of how chemical reactions occur, involving the breaking and formation of chemical bonds.

* Reaction forces in structures: This refers to the internal forces that develop within a structure in response to applied loads.

Please provide more details so I can give you a more specific and helpful answer.