The way some older cars do it is to rev the engine, then switch the spark from the cylinders to the tailpipe. The engine is effectively off, but still making revolutions, and raw gas is going though the process without being burned until it hits the tailpipe.
This is for old carburated engines without cats. This mod shortens the life of these engines.
That's the way it was explained to me. Also, if a person does it that way, I guess it's a good idea to make DAMN sure there's no leaks in the exhaust system.
I thought about doing this to my Mercury, but I just don't see a lot of people doing this anymore. There must be a reason for that. The thought of my old car burning down makes me sick.