DUO combines the power of air pressure and the gentleness of vacuum to give you the fastest most powerful self contained supplemental brake ever. 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. In addition, DUO is the pioneer in user friendly operation; Just Turn it on...and Tow.