Engine braking is a process of slowing down a vehicle by using the retarding forces within an internal combustion engine, instead of using additional external braking mechanisms such as friction brakes or magnetic brakes. When the accelerator pedal is released, the air intake valve is closed, and a vacuum is created, which prevents airflow from getting to the cylinders. This creates a high manifold vacuum that the cylinders have to work against, sapping much of the potential energy out of the system over time and producing the majority of the engine-braking effect. Engine braking is more effective in cars with a manual transmission, but it works with an automatic transmission as well.

Benefits of engine braking include reducing wear and tear on brakes, improving fuel efficiency, and giving the driver more control over the vehicles speed when going downhill, which is especially useful if carrying or towing a heavy load. Engine braking is not harmful to the engine, and engines are designed to run at thousands of revs per minute for hours at a time.