The compression brake, originally called jake brake which a trademark, as mentioned, in the early days was added onto the heads of a cummins engine. As I understand it it reverses the action of the valves to turn the engine into a compressor ( not really that simple ) there may be a small amount of fuel introduced to keep the cylinders from running dry.
I drove a 1962 Kenworth and we had one of the first 'jake' brakes installed. I could go down the g****vine grade with a 25 ton load and just touch the service brakes. Like I said in another post, this was the greatest invention ever for the heavy truck industry.