The Brawl Crawl: What it really is.
Shield Walking.
It might seem like a strange idea at first, but think of when it was being performed in the trailer. Pika crossing the raceway with cars coming at him, Snake after a roll, and then a couple shots of Wario, Snake, and Pika using it.
Of course, we can't see shields in the trailer, but if you think about it, they would most likely be up. When Pika is crawling across, it's obviously slower than his dash, so why would he do it? He has his shield up around him as well and is crawling across more safely.
After Snake rolls there's the same stance that we see him crawling in a little later. So how does one get into this stance? There's not enough information to say with certainty, but probably after rolling you either hold a button or hold the direction you were rolling instead of smashing it again. One thing that can be said most likely is that moving shield positions probably won't be back since I'm thinking that you'll be crawling by tilting the control stick. Some other possibilities are that the C stick handles crawling left and right and maybe shield dropping through platforms.
It also presents another statistic for the character on top of all the ones already present in Melee, Shield crawling speed. You can see that Pika actually crawls at a decent pace, while Snake and Wario inch along more slowly. It might also bring up the question of how the shield will protect them, and it might not protect the extremities of snake as he's sprawled on the ground as well as it might protect Pika or Wario who aren't so lengthy, but that's sort of the same general rule when characters are standing as well (except Pika's shield doesn't protect him anywhere near Samus', Peach's, Sheik's, Marth's, and basically anyone besides DK, Bowser, and GW in Melee).