Swapping the polarity has 2 effects:
1. The heat transfer is 2/3 to the + side and 1/3 to negative. Changing the polarity to EN will put more heat in the metal and less in the wire. So it will have greater penetration and less wire burnback.
2. The polarity effects the formation of droplets on the end of the wire. The combination of the charge of the drop, polarity and magnetic field control how the drop is formed and transfers to the work.
I think it also has to do with the gas. With gas MIG, the gas is charged at the tip. With gasless it is created at the work and travels in the opposite direction.
I tried reversing polarity to try to get better penetration on my little Clarke. Ended up spattering all over.