electrically or mechanically the boost bypass SOLENOID seems to stuff up regularly for people, by leaking boost pressure past the solenoid & reducing their boost pressure when it is not required.
Ok basically the M90 supercharger has a bypass valve in its inlet. It allows air to circulate around from the discharge pressure side of the blower & back to the inlet side of the rotors via the small butterfly valve. This needs to happen when the Throttle Body is in the closed position. With the Throttle Body plate in the closed position & limited air from the Intake Air Control (IAC) able to enter the inlet tract, the spinning rotors will still try to paddle air forward as the M90 is a positive displacement pump. So with a bypass valve open the air pushed forward can recirculate back up into the inlet tract for the rotors to push it through again & again, around & around. Because at idle the engine is not using (or needing) the amount of air the rotors are pushing forward, there is enough lower inlet manifold pressure to return via the bypass valve to the inlet side of the rotors. Yet still have the inlet side running at enough vacuum without cavitating (rattling & heat production). The condition at idle with the Throttle Body being closed & the blower still trying to paddle the air inside it forward. Would, without a bypass valve, create a large negative vacuum pressure, (positive inlet air pressure starvation) on the inlet side of the rotors, & cavitation (an impolsion) rattling will occur. If you didn't have a bypass valve in play the air in the blower would also heat up unnecessarily due to the resulting cavitation.
Now the Boost Bypass Valve is operated via a mechanical means. It has a air (or really a vacuum) operated diaphragm actuator hooked up to it. There are two sides to the actuator which are divided by a diaphragm, a upper (Top) chamber with light spring & a lower (Bottom) chamber. A Inlet vacuum hose is hooked up to the upper side (Top) chamber. At idle when vacuum is highest & throttle bodys are closed, vacuum in the upper chamber pulls the diaphragm up against the spring, which lifts a rod linkage connected to the Boost Bypass Butterfly Valve. The Boost Bypass opens and circulates the air & reduces boost pressure in the Lower Inlet Manifold.
As the throttle body is opened, the rpm picks up & the inlet manifold vacuum drops. As the vacuum drops the spring in the Top chamber takes over and pushes the diaphragm back down, returning the Boost Bypass Butterfly valve to its closed position (full boost).