According to the book the negative side gets more heat into it. My guess is with normal wire you may as well put the heat into the weld, but with flux cored the polarity is swapped so the extra heat can be used to vaporise the thicker wire and flux.
I tried some experiments with a gasless welder and didn't notice a lot of dirrerence from reversing the polarity, so I suspect the effect is subtle.
Hi Malcolm,
My mate borrowed my welder ( Cosmo mig170) he used plain wire and gas but never swapped cables. The fuse blew every time the shroud touched the metal, the wire burned into the nozzle and the weld was like dry dash. There must be something more about the Polarity. john,