To answer ur 'HOW' in the topic title here we go, this is my understanding of the system...as simple as it seems, it actually is, when you're driving along, your foot is on the throttle however much, and the car is using fuel, it same for while reving the car, however the car is not moving its stilling sucking fuel on rev.
When you let off, you cut off the air supply, so for a brief moment, the car will suck a extra slurp of fuel from the carby, or the EFI will still inject more fuel than necessary before it reacts and leans out. So the engine runs rich, meaning unburnt fuel finds its way into the exhaust.
So you have a exhaust full of unburnt fuel, oxygen, and heat. Flame thrower kits come with a spark plug that goes on the tip of the exhaust and sparks with help of a coil when u press the button, Combustion happens in the exhaust and bang! Flames come out.
Same theory for backfires, only no big flames in that case, just some loud bangs....
The kit only has the coil and a standard spark plug with wiring harness to operate....u can make ur own if u know wat u r doing....not too hard
Sent using eBay turbo at 40psi