panty vac

panty vac is a setup with baffled pickups in the valve covers connected to one way smog valves like chevys use threaded on bungs connected to the header collecters so that block pressure is sucked out by the exhaust instead of contaminating the intake charge thru a pcv valve. I am not sure if exhaust opens the valves with vacuum or the block pressure opens them. this is why i,m worried full exhaust might have more back pressure keeping it from working.