As you probably know, with the engine running the pressure inside the intake air distributor behind the throttle valve is always lower than the surrounding atmospheric pressure, which is why we commonly -- but inaccurately -- refer to it as "vacuum." As these diagrams show, the pressure is lower when (A) the throttle valve is closed and only a little air is drawn in through the by-pass, and rises (B diagram) as the valve is open and much more air comes in.