I started to write a simply reply, but the more I think about it the more some d!ckhead is going to jump up and down about the technical workings on the ECC, or charged air hitting butterfly on the plenum.
So, it’s easier to say when you vent the metered air to atmosphere, the value of Air entering the combustion chamber is less then what’s expected. So fuel is metered and expecting, lets say X amount of air. But it actually has X-n amount of air; n being the amount of air vented to atmosphere.
The picture is the wiring of the ‘powertrain control module’ of the Barra240T, which should help you reference what im on about.