From the site:
BIW writers decide on their own goals and submit them to the designated community thread the week prior to BIW. During BIW, writers write as much as they can and post their daily totals to the group. To stay motivated we recommend checking in everyday whether you write or not. We’re here to help with the support and accountability. This group works on the honor system.
No editing, no going back over what’s been written. Write, write, write. What is important is getting the words down, creating a first draft. Editing and revising comes later. Allow yourself to write quickly and without worry. Get your ideas down first.
Some members actually finish a book in one week and for others the only time they write is during BIW, thereby giving BIW a double meaning.
Writers can work on whatever they wish — a story, an article, book length fiction. It should be in the beginning stages rather than something you have already written and want to polish. You can even select something from work or school, like a research paper, as long as it is writing related.