The Jake Brake permits large trucks to descend long, steep grades at a controlled speed. It was the first practical mechanism for altering on demand the valve timing on a truck diesel engine, thereby converting the engine to a power absorbing machine. The modified engine can continue to power the truck in normal operation, allowing service brakes to remain cool for emergency situations. Invented by Clessie Lyle Cummins (1886-1968), this device (produced by the Jacobs manufacturing company since 1961) has contributed significantly to highway safety.