In theory it's like how a school project is scheduled - you have milestones and a deadline but you can work flexibly as long as you hit your targets. So if you want to get it all done on the first day, crunch on the last, or spread it out, it's up to you and no one cares as long as it's complete with sufficient quality.

Likely this doesn't work well on a team, or for more open-ended work. Never seen any organisation aside from private contractors who actually do this though.