[mr0077]

The PCV is normally open when the engine is off due to a weak internal spring and gravity. When you start the engine, manifold vacuum pulls the shuttle valve inside the valve housing closed, and allows a slight metered amount of air into the intake. When manifold vacuum drops (such as when opening the throttle under load), the internal spring pushes the shuttle open to allow a greater flow (hopefully of any blowby plus fresh replacement air from the other valve cover) thereby allowing the crankcase to be ventilated. So, at idle the PCV is essentially closed, and passes very little air; it only passes a significant amount of air and vapors when the engine is off idle and under load.

When you pull the valve out of the valve cover (or wherever) while it is hooked to an idling motor, the manifold vacuum has the internal shuttle pulled into the "closed" position, and it will not rattle. Put your finger on the bottom of the valve housing and you cause the vacuum to be the same on both sides of the shuttle (equal to manifold vacuum), so the spring pushes the shuttle into the open position, allowing it to rattle when shaken (that's the "shade-tree" method to test it). When you take your finger off the bottom of the valve, air flows toward the manifold, the engine vacuum pulls it shut against the weak spring pressure, and it won't rattle. All this assumes a properly functioning PCV valve.