A Jake Brake is an engine brake that helps slow down heavy trucks. It opens the exhaust valves near the top of the compression stroke, releasing the compressed air from the cylinders. This turns the engine into an air compressor, creating drag that slows the truck down without using the service brakes. It makes a distinctive, loud rattling sound that can be heard from blocks away.
Using the engine brake is vital for control on steep mountain descents. It keeps the service brakes cool, preventing them from fading or catching fire. Many towns ban the use of Jake brakes within city limits due to the noise they make. Drivers must turn the engine brake off when roads are icy, as the sudden engine drag can cause the drive wheels to lose traction and slip.