no pop, no flames: stock exhaust & tuning
pop but no flames: exhaust and lean condition on overrrun (e.g. aftermarket exhaust but no fuelling adjustment, would leave you a little lean, gives plenty popopop)
pop + flames: exhaust and fuelling adjustment giving you a mixture of lean overrun and extra/unburnt fuel/running rich which then = flames (aftermarket exhaust + fuelling adjustment)
as far as I'm aware going somewhat lean on overrun (when you let off the throttle after beign on it) is what caused the poppopopopop but you can also end up with some unburnt fuel in there if you're running fuelling adjustment.. so for example if i rev hard and let off and let the revs drop slowly it does popopopopopop but no flames.. but if i slam the throttle open and closed or rev it hard or to the limiter where it goes from 100% fuel to 0% to 100% to 0% really fast (i think) you end up with a mixture of the popoopop (lean) on overrun and flames on full when the additional fuel is added in (rich) when slammign the throttle fully open again. I guess you could say the popping and flames are an expression of the confusion between the two engine states.
More highly tuned engines are also likely dumping more fuel in at full throttle so any popping and flames might be more exaggerated because there's just more fuel in play, whcih is why mad drift cars with massive turbos etc will shoot bigger flames bc they have sh!tloads of fuel in play so that difference between full throttle and 0% throttle is even more noticable.
idk if that's 100% correct but that's what seems to happens on mine when it does it. I tried filming the flames but it's difficult to get them to show up :-\