When the hammer is down, the interface between the rebound slide and hammer will prevent the latter from moving forward enough to contact the firing pin (frame mounted pin) or from a hammer mounted firing pin to contact the primer.

Centennial model revolvers do not use the hammer block as the rebound slide by itself renders the gun drop safe.

The hammer block does protect the gun from firing when the hammer is cocked for single action use. In this case the hammer block prevents firing unless the trigger is pulled fully to the rear.