You can't get the vacuum anywhere high enough in your mouth to simulate 12-15-18 in of vacuum and sense the flow change. You're probably just opening the PCV.... it is also a check valve to prevent flow from the intake into the crankcase. It would be bad idea to let the flow go the other way and allow the crankcase to suck in a fuel air mixture ready to burn! LOL What you are doing is an old test to see if the PCV is stuck closed.