The variable vane turbocharger design used in the 2004.5 - up Duramax engines to reduce turbo lag and increase power can also help slow you down. BD utilizes this technology to electronically close off the vanes which can create up to 130 retarding horsepower. This braking force will help to maintain your speeds on descents and curves, giving you better control of your truck and load.While in TowHaul mode, switch BDs Variable Vane Brake (VVB) on and every time you let off the throttle, it will close the variable vanes in the turbo, developing exhaust back pressure in the engines combustion chamber which creates resistance against the piston from reaching top dead center.The VVB rotary switch offers three different modes included automatic moderate and aggressive downshifting programs for the Allison transmission to give better weight control. Exhaust braking also works in Allison manual shift selections. The VVB integrates with the cruise control to automatically accelerate and decelerate to maintain vehicle speed.Easily and simply installs by plugging into existing factory ports and by mounting the rotary switch on your dash. All of these features in one package which with some competitors would require two separate ones at nearly twice the price and twice as long to install. Now that is value added engine retarding at its engineered finest.