The supercharger must spin freely with the belt off or it will be a major restriction and not want to rev up past iirc 2500 rpm or so. Under high vacuum(low throttle) the engine will breath through the bypass valve. With low vacuum(high throttle), the bypass valve will shut and the air must move around the rotors. the rotors will spin at higher rev but they are still a big restriction because they are only letting air through as they spin.