The PVC valve must prevent backflow so that a backfire will not ignite the crankcase gases (a great way to loose all your gaskets). Under normal flow, the PVC valve actually has a calibrated flow curve. Max flow is desired under heavy loads, where blowby gases are high. At low loads or idle, there is little blowby to worry about, hence the lower flow of the valve. This is also why it is important to use the correct PCV valve for your engine.
