I am trying to get a handle on this myself; the couple attempts I've made haven't been very successful, but I learned a lot!
As much as I love the book, things aren't always clearly explained. And while the pics are lovely, there are also discrepencies in the order some are shown. I've found it very confusing, but the more I have at it the more it's making sense. One thing for sure is that I think their dough is very wet for pizza, at least for a "starter noob" like myself to handle. It's a whole different game with a dough that loose, I think it's something you learn to get comfortable with over time. I plan to adjust the hydration a little next time I try...
Side note, I was fortunate to go to Tartine last month on a short trip to SF! It was pretty cool I must say, neat to see Chad and employees in the book. I took a lot of pics!
If you look up a guy named "pizzahacker", he was one of the Tartine testers and has made a niche for himself making Tartine-based dough pizzas in a modified Weber grill. In the video on his site he says he uses some commercial yeast along with the starter. hmmmm..... Unfortunately he wasn't appearing anywhere until later in the week, so I didn't get to meet him or try his pies. :-(