[MENTION=382]airjor13[/MENTION] its meant to be a very simple yet super effective method of venting positive crankcase pressure, especially on boosted applications.
Here's a link detailing the negative effects of crankcase pressure.
http://www.shophemi.com/images/media/p-2273-arrington_ccv_bible.pdf
The factory PCV valve flows one way, to prevent boost from entering the crankcase. This means that under boost the excess pressure will work it's way out through the clean side pcv (thus entering the intake) or a seal (CTSV's are notorious for leaky rear mains).
Our breather simply replaces the oil cap (10 second installation!! lol) and facilitates venting ONLY under boost via the one way valve. Under vacuum (which is when the PCV kicks in) the ball seals preventing unmetered air from entering as well as allowing 100% function of the factory pcv. This is why we recommend running a closed system oil separator (Such as the Mishimoto) on the PCV/ "Dirty" side. The breather and catch can both work under 2 different conditions, venting excess pressure at boost, and drawing out crankcase contaminants under vacuum.
Let me know if you have any other questions!