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A BPV/BOV has an inlet and an outlet. The inlet is on the boosted side of the intake, between the turbo and the throttle body. The Outlet side is on the ambient/vacuum side of the intake, before the turbo (and after the air flow meter in certain setups like this one). The poppet/valve is held closed by a spring and depending on the type the spring is assisted by the boost pressure. When you let off the throttle the intake plenum (the portion AFTER the throttle body) pressure goes negative and the manifold reference (the small vacuum line) goes negative sucking the poppet/valve open against the pressure of the spring. The strength of the spring dictates how much vacuum is required to open and keep the valve open. Excess pressure between the turbo and the throttle body is now discharged back to the intake before the turbo. This prevents new air from entering the system and the ECU doesn't add any more extra fuel... you don't run lean though because the ECU has already measured and allocated fuel for the air which has just had its trip extended. If you just dump the air out, the ECU still thinks it's in the system and it still send fuel for it.