Pic 1
Pic 2
There is a solenoid on the outside of the blower(seen attached to the blower in pic 1, on the lower right side, also top right of Pic 2). It has two vacuum ports (you can see both in Pic 1), one to atmosphere and one to pre-blower vacuum. It also has an electronic connection controlled by the PCM. It rotates a plate ("valve", gold piece in Pic 2). It connects the pre-blower(after throttle body and before blower rotors) and post blower (see the blue circle piece sticking out the left side of the intake manifold in the first pic). When it opens, it allows post blower air (from the intake manifold) back up to the blower inlet (before the blower) allowing less or no boost to be made. As RPMS increase, the valve will close, allowing more boost to actually get to the engine.