2 valves in front and 2 valves in the back operated by electric motor. The valves are connected to the motor with cables and special unit controls the motor.
In the automatic mode, the valves are opened or closed by the RPM or automobile speed. For low RPM(Stage 1), all the valves are closed which provides quiet sound. For mid RPM(Stage 2), only the back valves are open for both power and quietness is provided. And finally for high RPM(Stage 3), all the valves are open for the most power. Also the manual mode let you open all the valves.