sequentially opening and closing the exhaust port of the combustion chamber and the secondary hydraulic circuit is deactivated with respect to the secondary piston assembly in this operating mode the force imposed at the top of the primary piston 40 at the pivot surface is transmitted through the primary piston assembly to the latching balls 70 which are trapped against hard stop surface 142 the lower slope 120 of the control piston contacts the blocking balls in this hard stop condition the force component generated by the exhaust valve actuation reaction force will keep the venting valve 116 closed it should be appreciated that an alternative to the illustrated one piece control piston with integral valve 116 could equivalently be implemented using a control piston with captured ball valve member at the top the lower slopes 120 of the control piston adjacent the apex or hilltop of the acute angle formed by the upper and lower slopes does not provide a positive downward force against the blocking balls but rather merely contacts the balls to assure that they maintain their positions laterally outward against the lower curvature 142 of the detents 68 while resting on the shelf 90 when the secondary oil gallery is pressurized thereby pressurizing the control chamber 138 the control piston 112 separates from the vent seat 126 and begins moving downwardly against the force of piston spring 132 while the roller of the arm 20 travels.