The above explanation was good, but let me try another aproach.
The PCV valve is a one way valve. It allows flow in one direction only (there is an exception, but for the sake of this discussion, we won't go there yet). The flow is into the intake manifold, usually at a point that provides even distribution into all cylinders. The flow is from the crankcase. The purpose of the PCV is to ventilate the crankcase. Before PCV systems, this ventalation was from a road draft tube, which was ineffective below speeds of about 25 mph. The road draft tube also allowed fumes into the atmosphere and the PCV captures them. To pull the fumes off the crankcase, fresh air must enter from another hose. This hose is connected to the intake prior to the throttle body. Air circulates through this tube, into the crankcase, into the tube with the PCV valve, and then into the intake manifold so that the fumes are directed into the combustion chamber to be burned. Where did these fumes come from? They are unburned air / fuel mixture that got past the rings (called blow-by). Even an perfectly operating engine has some blow-by. So the PCV system is keeping the fuel from the blow-by from contaminating the engine oil and sends it back to the combustion chamber.
All of this is well and good as long as there is suffecient vacuum to scavenge the fumes. During low vacuum events, the blow-by will re-enter the intake from both hoses, on both sides of the throttle body, but still not allowing them loose into the atmosphere.
By the way, the PCV system was the first smog device. It came into wide use in 1961 (if you really wanted to know). The very first PCV systems did not have the return hose to the air filter area. The closed system as we have it today came a little later. Converting to a closed system by adding a hose became a required modification on title transfer on cars that were built with an open system.
Probably more than you wanted to know.
