Surface deformation takes place when the impact body hits the test surface, which will result in loss of kinetic energy. This energy loss is detected by a comparison of velocities vi and vr when the impact body is at a precise distance from the surface for both the impact and rebound phase of the test, respectively.

Velocity measurements are achieved through a permanent magnet in the impact body that generates an induction voltage in the coil of the impact device. The signal voltage is proportional to the velocity of the impact body. Signal processing provides the hardness reading for display and storage.