[Mr Jody]

In addition to this being evidence of an overly rich mixture, it could also mean that the spark timing is possibly somewhat retarded (occurring late) when you come off the gas, which gives spark to that slower burning rich mixture as it is exiting the exhaust valves. In addition to checking the mixture on deceleration, you may also want to check the spark timing to make sure it is sufficiently advanced, or that it is not allowed to drop off too fast as engine speed drops.

While it may look cool, I think there is a potential for damage to your sidepipes and headers, or at least to the associated gaskets. The same force of that explosion that you see coming out of the pipes is also going the other way, back up towards your engine, as your combustion chamber is now the thin wall of the headers and sidepipes instead of within the confines of the much thicker cylinder walls. You may have heard of some passenger cars blowing up mufflers in this manner.