An engine usually only backfires when it has been run quite some time and is hot, then letting it idle creates more heat along with, at idle an engine runs richer so more fuel is entering a very hot muffler and when the engine is shut off, all that gas ignites in the muffler thus, a backfire is created.
Now at full throttle, the engine is running at its best and is in a leaner or optimum fuel mixture mode, thus no excess gas is entering the very hot muffler, so no backfire happens.
You will also find that if you start... and shut off your engine at full throttle, your results will be much better all the way around.
Try this....you will see!