It is a butterfly valve like the one in the throttle body (see picture). The default is closed. When vacuum is high (idle-cruising) an actuator opens the bypass valve so that intake air bypasses the compressor. When accelerating, vacuum decreases and the bypass valve instantly closes. Intake air now goes through the compressor, building boost. More power means more fuel means larger injectors. However, in order to pass CARB, the base kit had to use the stock injectors.