flames out exhaust?

http://files.automotiveforums.com/images/igor_sushko_gt-r_willow.jpg

anybody know how this skyline is burning flames from the exhaust? now, i've heard a little about his, but wasn't too sure, some boosted cars that vents to atmosphere will cause backfire, and some cars that just naturally cause backfiring thus the flames, either than that, i've heard of the kits, but i doubt this skyline was using a flame kit, greatest appreciation

downshift and running rich momentarily with no cat.

Some sewer gas got caught in an atmospheric pocket and was ignited by the velvet mofia.

lol
in high perfomrance vehicles not certified for road use that are using enough fule to do such a thing....
when you let off the gas for a down shift the engine goes rich and not as much fule is burned, the vaporized fule lingeres in the exaust system near the tail pipe, because the flow is also greatly reduced, when the driver gets back on the gas that blast of hot gas is hot enough to ignite the un birned fules, causing a flash of flames...
My go-kart used to do this, only ir ran off of an alcohol mix so the falmes where pretty blue, fun to watch at night.