Why it backfires when you slow down on a Honda Cb110?

when you slow its effectively cutting off air to your engine which slows how fast the piston burns fuel, so when you do this more fuel then air gets in then it should then it doesn't burn properly because the fuel/air mix is out and it will go through into your hot exhaust system and then burn and create a backfire it is not bad in any way for your engine or bike, unless its damaging the paint or burning your passengers legs off!