Product Description: Demco 9599006 Stay-In-Play Duo Towed Vehicle Braking System Inertia-base towed-vehicle brake system. Simple to control, easier to use, and the smallest actuator in the industry. Duo's small operating unit is installed under the hood so you never know it is there. Duo uses the power and speed of air pressure to activate your towed vehicle brake pedal while creating vacuum for the power brake system. These two features combine to create the only progressive and proportionate vacuum assisted system with total control. Duo requires BOTH deceleration AND coach brake lights to apply the towed vehicle brakes. By combining two separate signals the need for complicated electronics is eliminated, which provides you a more reliable, long lasting system.