Most private trackers monitor how much
users upload or download, and in most
situations, enforce a minimum
upload-to-download ratio.
How exactly can a tracker figure out how much data was uploaded and downloaded by each user?
My understanding is that a BitTorrent tracker is merely a registry of users that are currently downloading/seeding and that peers, once connected, transfer data directly. So I wouldn't think that the tracker would know anything about the amount of data transferred, much less, where it came from.
Best Answer
The BitTorrent clients report back to the tracker what they are doing. There are some clients that lie but this can be detected by the fact the reports from the two sides consistently disagree. That's why you'll see some site banning the lying clients.