a restrictor in each hose and a wire bristle looking thing. If the restrictor gets clogged then the engine has no where to pull crankcase breather air from. The pcv's allow air into the plenum based on valve postion and vacuum/pressure.
When the pcv's are open while the plenum is under vacuum, if the fwd hoses are clogged or restricted the vacuum draw will pull more oil laden vapor from the crankcase. Your compression numbers are good so in your case blow-by is minimum and under normal driving blowby will not subsidize the lack of a working breather system.