The PCV valve "closes" (goes to minimum flow) at high vacuum (such as idle conditions), and "opens" (max flow) when manifold vacuum drops off, so the amount of flow through the valve decreases at idle to avoid upsetting the idle mixture, and increases at operating RPM, when the relatively small amount of air flow won't have a major effect on the fuel/air ratio. It will pull vapors with
oil and other combustion byproducts from the crankcase into the intake manifold...they get burned and exit through the exhaust, with no significant impact on power output I'm told (unless the engine has so much blow-by it is already down on power). The PCV valve is almost always a win-win item for non-race motors IMHO.