A secondary braking device that creates exhaust back pressure in the engine slowing down the cylinders and in turn slowing down the driveline.

Since diesel engines lack a throttle valve on the intake manifold, there is no intake vacuum when the engine is not fueling. The intake vacuum creates the slowing effect felt in gasoline engines when they are going down a hill with the foot off the gas. Many different strategies are used on diesels, the least expensive (and also one of the least powerful) of which is the exhaust brake.