Air is pulled into the device when the moving scrolls are at their furthest from the fixed scrolls, leaving a large gap. As the shaft rotates the gap closes, and the point of widest opening rotates to a point further "into" the charger. As the process continues the gap eventually ends up on the middle of the housing, at the point where the higher pressure air exhausts into the engine. The scrolls are shaped so the gap slowly decreases in size as it moves to the centre, thereby compressing the air.