As I had said it just like an emission air pump which injects AIR into the exh to ignite any unburned fuel and cut down on emission's. Not everything burns in the cyl on ign so if any air is pulled into the exh on decell it can ignite the unburned fuel sorta like what a diesel does. Now of course if the car runs to rich it can make it worse but if there is no air pulled or pumped into the exh then it can be way rich and still wont pop on decell if the car runs an exh system and not open headers. It could have had a small leak that was not enough to cause a poping but if you change the mixture then that small leak may be enough to cause the poping on decell. And you also slam the throttle shut on decell which cuts off the air going into the eng on decell. There are other things that may cause the poping in the exh on decell but 99% of the time it is an exh leak. Ron