Back to 0kb, then it'll see the downloaded files and resume from there, IIRC. I could be wrong, I've only had to do it once.
It only tracks what it has to download, not what it has downloaded in the past but not installed. So the metric in the downloader only tracks what it has to download in the future.
Example: Your update is 5 gigs, you stop after two and restart, the downloader will start at 0 but it will show only 3 gigs remaining to download.
