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It's really not complicated.
Back in the old days, you had a "road draft tube" that vented the CC at idle and created a slight vacuum cruising. (Bernoulli Principle)
As emission controls started to be added, the "closed loop" or "Positive Crankcase Ventilation" system was used to help prevent HC emissions.
The way it works is under high manifold vacuum it sucks fresh air from the intake tract
(hose to the intake tube) that will go to the crankcase.
The other side has a spring loaded "metering" valve. Again, when manifold vacuum is high enough, it sucks open the valve and the fresh air/unburnt HC, etc. go into the manifold to be reburned.
If there isn't enough vacuum (WOT), the valve shuts and the air rushing thru the intake
creates the vacuum (Bernoulli Principle) and the flow reverses. (There is no fresh air going into the CC during WOT. It is under vacuum only)
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