I've always see these on cars, but I've known how they work, other than putting in a spark plug in the exhaust. I checked out a video install (Stacey David, from an old episode "Trucks")

He said when you turn on the switch, it cuts out the engine spark, directing the spark to the exhaust, which burns the fuel thus creating flames. But, he said,... "this washes down the engine cylinder walls and rings, so change the oil often".

Can damage be done ?