The reasons that he gives on the site are OK, but there's lots of different ways to do things. Some kinds of MIG work one way, and other kinds work the other way.
If you're using flux-core wire with no gas, then most recommend DCEP. The way you hold the gun and move it also has a lot to do with it. On smaller (non-industrial) machines, you are better off to crank up the heat near maximum, and slow the wire speed down till it makes a smooth "humming" feeling. Try to aim the wire straight down into the hot part, or even tilt the gun back so the wire points back at the already-welded area, into the middle of the puddle. This is so that the heat from the weld partially melts the wire and it doesn't need so much electricity to burn down a cold wire. It should be like "dragging" the weld along.
If you're using a solid wire with gas, you can "push" the weld ahead, aiming into the unwelded area slightly. You get slightly better penetration that way. Once you have the machine setup properly. With hard wire and gas, you can set the polarity, etc. up either way depending what you want. Hard wire and gas is a very versatile way, and I recommend it.
In all cases, you should keep it absolutely clean. This means grind your parts shiny. If you have to go over a weld or next to it, you should make that shiny too before you re-weld it.
Hope that helps!
