Here's why:

* Newton's Third Law: The fundamental principle at play here is Newton's Third Law of Motion, which states that for every action, there is an equal and opposite reaction.

* Action and Reaction: In a rocket engine, the action is the force exerted by the engine on the hot gases, pushing them out the nozzle at high speed. The reaction is the equal and opposite force exerted by the hot gases on the rocket, pushing it forward.

* Momentum Conservation: This exchange of forces also obeys the principle of conservation of momentum. The rocket gains momentum in one direction, while the hot gases gain momentum in the opposite direction.

In short, the action force of the rocket engine directly pushes the hot gases, and the reaction force from the hot gases is what propels the rocket forward.