well...lets see if i can explain it
a "jake brake" is actually a trade marked name for an engine brake used in Diesel engines. Mostly 18 wheelers have them.
Some diesel pickups- mine included have an exhaust brake.
when a trucker lets off the gas- the engine/exhaust brakes- thats the loud rumble you hear.
Some towns/cities tried a few years ago to ban them, HOWEVER that said- it was found by supreme courts, that they are safety features. So truckers dont run outta air, in their air brake systems, they rely on these brakes.
Hope that was clearer than mudd.
Brandy