A real "Jake brake" (Invented by Clessie Cummins) slows a truck down by turning the engine into an air compressor. It does this by modifying the exhaust valve t

A real "Jake brake" (Invented by Clessie Cummins) slows a truck down by turning the engine into an air compressor. It does this by modifying the exhaust valve timing to open at top dead center while cutting fuel. Once turned on all you do to engage it is to take your foot off the accelerator. That staccato popping sound is the compressed air blasting out of the exhaust valves. On old rigs like my '61 Mack the jake brake is switched manually. You have to be sure to turn it off once you get close to engine idle rpms otherwise the engine will stall. A quieter version called a bleeder brake which does the same thing though it leaves the exhaust valve slightly open during the entire compression stroke eliminating the popping sound. There is also the exhaust brake which puts a butterfly valve on the exhaust line creating back pressure when closed. Though the exhaust brake is known to cause problems with some valve train setups.