What (who) is jake brake - definition

A compression release engine brake, compression brake, or decompression brake, frequently called a Jacobs brake or Jake Brake, is an engine braking mechanism installed on some diesel engines. When activated, it opens exhaust valves to the cylinders, right before the compression stroke ends, releasing the compressed gas trapped in the cylinders, and slowing the vehicle.

Clessie Cummins was granted a patent for the engine compression brake in 1965, and the first company to manufacture them was Jacobs Vehicle Systems. The terms Jacobs brake and Jake Brake properly refer to compression brakes manufactured by Jacobs, though the terms have now become generic trademarks, and are often used to refer to compression brakes in general, especially on large vehicles and heavy equipment.