Virgil; The engine brake works through the transmission to apply braking effort to the rear wheels. Going to a lower gear gives the engine more leverage, so to speak, to hold it back. All a Jake brake does is essentially make a large air compressor out of the engine. The Jake delays the opening of the exhaust vales until TDC or shortly after.
The "POP" you hear from them is the explosive release of compressed air similar to sticking a pin in a balloon. With the newer Jakes and engines, up to 97% of engine horsepower can be converted to braking effort. A 500 HP Cat will have 485 HP of braking effort at 2200RPM and that will be multiplied through the transmission proportionate to the gear you're in.
There are electronic retarders that are basically a dynamo. An armature is fastened to the drive shaft; a field coil is wrapped around it and secured to the frame of the vehicle. The control on the dash is nothing more than a rheostat that sets up an electric field in the armature and slows the vehicle. I used them on VanHool buses but they're not as efficient as Jakes.
