Exhaust gas that exits the exhuast manifold have two paths they can go. Usually they flow into the turbo fins and spool the turbo, once a certain psi of boost is achieved, the wastgate opens and the exhaust gas has another exit to flow thru. Since the exhaust gas bypass the turbo when it goes thru the wastgate, the turbo doesn't make as much boost. A wastgate function is to regulate boost by manaing the amount of exhuast gas flowing thru the turbo. A bov/bpv has nothing to do with exhaust gasses.