in an internal combustion engine using poppet type valves , a center pivot rocker arm is moved through a specific path wherein the roller in contact with a cam is moved to alter the phasing of the valves or injectors in the engine . depending on the interface between the valve or injector and the rocker arm , the rocker ratio of the rocker arm may be altered , giving a change in lift as well . by positioning a control arm at desired points on either side of a centered position , and rotating the control arm about its own pivot point via an arm actuator , phase change is achieved . the arm actuator controls the location of the control arm and thus the timing of the valve or injector relative to the rotation of the cam . advanced , centered , and retarded phase change is possible depending upon the movement of the control arm .