let's it suck fresh air as the exhaust goes out the pipe - real lean/hot bang it goes

that's my understanding of a leak backfire - a very rich one can happen as well with fire running out the pipe - where you get raw fuel puddle in the muffler, or course the best ones are from a missing engine - no spark for a few rotations and you get a very nice mixture in the pipe and then a good spark - whooo boy you can get one that blows things apart - saw one like that in a motorhome a buddy had - the hot on his battery bank had shorted against the frame rail so most all the juice was gone - he hit the electric starter on the aux generator well it kill the 440ci engine as soon as he let up on the button the engine fire back up - blew the muffler in half on the winbago