To reduce the emission of unburned hydrocarbons from the crankcase to the atmosphere, with a sealed engine, blow-by gases and oil vapor are drawn from the crankcase through a wire mesh oil separator into the engine air supply path and are burned by the engine.
With high reservoir depression (idling, deceleration) gases will be sucked out of the crankcase. With low reservoir depression (acceleration, wide open throttle) gases exit the crankcase at a relatively higher crankcase pressure and, if the engine is worn, high crankcase pressure (due to increased gas breakthrough) will cause the flow to return to the collector.