Its an old hot rodding trick. Started out with carburated cars. Run the engine till the exhaust got hot, shut off the engine, pull the choke and quickly start the engine. The rich exhaust vapors would ignite in the hot exhaust pipe shooting flames out the tail pipe.
Later, people started putting spark plugs in their tail pipe so that they didn't have to shut the engine down and turn it back on. Later still, people would retro fit old army flame throwers to their exhaust to REALLY shoot flames out the back.
Today, you see unburned fuel in the exhaust of turbo charged cars ignite in the exhaust. Since race cars run a slightly rich mixture and the exhaust for turbo cars run so hot, flames shoot out the exhaust when the driver lifts off the throttle.