This bit of advice comes from a British author who forces himself to write the entire draft of a novel before revising a single line. Of course, not every writer works that way. Nor is it always the best strategy for academic papers, where you may need to rethink the logic of your argument as you work.
Still, it is an important strategy to try, particularly if you're someone who gets bogged down obsessing about word choices. Word processing has, unfortunately, done a lot to nurture such obsessive tendencies. I started out doing college papers on a manual typewriter; and as much as I don't miss plunking out 20 pages in a night, getting progressively loopier and loopier from White-Out fumes, it had its advantages. Chief among them: it was impossible to look back. You had to keep moving forward.
This strategy can be particularly useful when writing a short reflective essay (2-5 pp.) or when tackling the first draft of a dissertation chapter. In the first case, the shortness means you'll be able to revise with relative ease, and there won't be a terribly complex argument to juggle.
When it comes to the dissertation chapter, writing it all out without revising typically won't yield a full rough draft. You may well find yourself having to write in brackets things like "Here's where I want to put the argument about impact of education on life span]." What it very well may give you, though, is a kind of crude but sturdy structure. Then you can begin slotting in missing data, pieces of argument, etc., in order to build the first full draft.
Fear probably kills more dissertations than anything. Every time you make progress, you fear you need to know more, that no one will buy your argument, etc.
"Don't look back" is a fear-busting piece of advice. Just keep writing it out as if your life depended on it. You know what you put down won't be polished or professional. But it will be there, and once there's something there, the rest will follow far more easily.