If the engine is running, the pistons are pulling in air. How hard they are pulling, can be measured with a vacuum gauge. High vacuum pulls the PCV plunger up to restrict the metering into the manifold to a minimum amount. At very-low vacuum the plunger drops down, and more or less closes the valve. At inbetween settings, the valve will be somewhere in between and could be jumping around.( You'll hear it when it does that,lol) With HotRod engines idling at 10 to 13 inches, this plays real nice with the PCV, as it can be used as your principal idle air bypass.