The valve spring exerts stress against the valve retainer to maintain the valve closed. When the rocker arm in a pushrod engine, or the cam follower in an engine moves and pushes the valve open, the strain exerted from the spring increases. A valve spring is placed around the stem of a valve and held in place by a retainer. Its main job is to control the entire valvetrain, ensuring that the proper amount of spring pressure is applied consistently to prevent valve bounce.