[Richard and Rhonda]

The jake brake uses the high compression of a diesel engine to provide resistance. The way it does that is that the jake actually opens the exhaust valve as the piston reaches the top of the compression stroke and lets the compressed air escape through the exhaust manifold. That is the noise you hear when on the jakes.

A transmission retarder actually has a fluid circulation loop in the transmission that is restricted and acts as a brake. Most transmission retarders have multiple settings depending on how much braking help you want. Almost all of them have a cooling loop for the tranny fluid because it does get very hot. They work very well, and with more adjustability than jakes, but you do have to keep an eye on the transmission temperature.