At idle, when the manifold is under high vacuum, the plug should be "sucked" inward into the body of the PCV valve. Like picture 1B.
The fuel air ratio curve of a motor is fish hook shaped, it is rich at idle, lean in the middle, and rich again at wide open throttle. The engine must be a little richer at idle in order to idle properly and not stall. If you let too much air into the engine at idle it will lean the mixture out, cause the motor to idle rough, possible stumble, and possibly stall. So although the plug is sucked deeply into the body of the PCV valve, the PCV valve is designed internally to either shut off air flow altogether, or to regulate it down to a small trickle.
The internal design of PCV valves vary, they sometimes have a mating face on the end of the plug you can't see, and the body of the PCV valve sometimes has a seat for that mating face to seal against. Observe the pic above.
(1) Is designed so it never completely shuts off (no internal seat, no mating face on the plug)
(2) Is designed to shut off but it allows a small regulated flow via a drilled orifice in the plug
(3 & 4) Are designed to shut off completely
I just read the emissions section of the '73 Ford Engine Manual and it says the PCV system is designed to operate full time when the motor is running. So you got me Mike, I'm "kinda" wrong. But the concept still holds true, if the PCV valve allows too much air flow at idle, the motor will idle rougher than it should. In that case the Boss 302 PCV valve is a known remedy.
Blow-by is at a minimum at idle, so shutting the PCV valve off entirely at idle is not a bad thing, unless you're a manufacturer trying to meet federal emissions regualtions.
Where the PCV system doesn't work properly is at wide open throttle. In that situation blow-by is at its maximum, yet intake manifold vacuum is so low that the PCV valve is almost in the engine-off position, it is not drawing much, if any, vapors from the crank case. The blow-by ends up blowing out the breathers. This is why race cars must use something other than a PCV valve to draw vapors from the motor (exhaust aspirators or vacuum pumps).
-G