in a specific embodiment a cam unit comprised of three differently profiled cams acts on follower portions or working surfaces on a shiftable first rocker arm to cause a second rocker arm to open and close a valve associated with the combustion chamber or cylinder of an internal combustion engine in accordance with the cam profiles and the contours of the follower surfaces to maintain the valve completely closed , such as during engine deceleration , vary the valve duration and produce a lesser valve lift to effect an engine throttling mode of operation , and vary the valve duration and produce a greater valve lift for a higher engine performance .