there are gases that build up in the head from combustion, the PCV merely lets them out. what is supposed to happen is that the gases are to be sent back into the intake where any unburned stuff can be burned during combustion and anything else can be passed through the exhaust to the cat and so on.

there are many disadvantages to this, becuase less air is getting in and whatever else you want to say, it's just not good sense to have anything but pure air and gas going into the cylinder for combustion if you are looking at it from a performance and even reliability standpoint--

so to recap this is what happens: A)relieve gases from the head B)send them back through combustion to be burned off/cycled through the cat/ etc.

in this system shmoo is relieving the gases but not recycling them to get burned off.