David - most likely yer engine is not equipped with an afterfire solenoid on it's carb - a relatively new feature on most new small engines these days.
It's function is to cut off all fuel flow in the carb's load circuit immediately when you turn off ignition. This prevents a significant amount of unburned fuel/air mixture from being expelled into the exhaust sys during engine spindown- where there are usually some red-hot carbon spots that ignite that mixture causing the bang that none of us really care for.
dropping yer engine to idle for a cuppla minnits before shutting down allows both those exhaust carbon hot spots to cool and to slow engine rpm to where it stops more quickly, minimizing the amount of unburned fuel/air presented to the muffler.
I wus lucky - grew up on a farm with the usual equipment along with a dad reknown for his leather lungs. I very quickly learned to let the tractor and other powered farm devices to idle/cool a bit before turning off - even tho the majority of them were watercooled and much less prone to backfire. Dad had a way of emphasizing such.
