The rebound hammer test method is based on the principle that the rebound of an elastic mass depends on the hardness of the concrete surface against which the mass strikes. The operation of the rebound hammer is shown in figure-1. When the plunger of the rebound hammer is pressed against the concrete surface, the spring-controlled mass in the hammer rebounds. The amount of rebound of the mass depends on the hardness of the concrete surface.

Thus, the hardness of concrete and rebound hammer reading can be correlated with the compressive strength of concrete. The rebound value is read off along a  graduated scale and is designated as the rebound number or rebound index.  The compressive strength can be read directly from the graph provided on the hammer’s body.
For more details on principle, procedure, advantages & disadvantages of rebound hammer test, kindly check the article below-

Rebound Hammer Test on Concrete – Principle, Procedure, Advantages & Disadvantages