[davepl]

When I was an idiot kid I used to do this intentionally. I once did it to a guy on a bicycle under an underpass... and you now know roughly how loud that must have been! You turn the ignition off, coast the engine, let it pump the exhaust full of fuel-air mixture, turn the key back on... BOOM. Though I somehow managed to never blow up my exhaust.

Anyway, now that I'm older and wiser, I'll just reaffirm what others have said here: unburned gasoline in the exhaust ignited.

Generally, you need some kind of a timing glitch to fire it off in the exhaust though. So there might be two different (or related) problems contributing to this one.