There are a few ways this happens,
Compression braking in a gas rig uses the engine to slow the vehicle, not very effective in a diesel UNLESS the diesel has some type of gate to create vacuum or backpressure. 12v cummins.....no luck, 7.3 up fords....luck.
There is also a retarding feature in some auto transmissions that use the converter to eat up load. Early transmissions had troubles trying to redirect load through different bands....fords would chew reverse band by down shifting manually. Allison retarder was/is awesome.
