It's the zip file your Nexus One will download automatically in the background during low bandwidth times of day (to avoid strain on the cell network). It doesn

It's the zip file your Nexus One will download automatically in the background during low bandwidth times of day (to avoid strain on the cell network). It doesn't do so right away, as they also want to stagger the rollout. If you do the manual download and recovery method, you're just doing what the update will do eventually.