Yeah, a Roots blower is positive displacement. Every rotation can move a certain volume of air. But, if the throttle is closed, it will not be getting any air to move, therefore no boost (just parasitic drag losses). The throttle must ALWAYS be the determination between boost or not. Otherwise how would one control the engine output? [emoji3]
During low load operation, the blower is just drawing power from the engine to mix the intake charge. Only during heavy load will the throttle be open enough to allow enough airflow to produce boost - a positive manifold pressure.
During low load operation, the blower is just drawing power from the engine to mix the intake charge. Only during heavy load will the throttle be open enough to allow enough airflow to produce boost - a positive manifold pressure.