When you visit the website, it will check you IP address automatically and present you with files it knows you downloaded. If it doesn't have your IP address in its database, then it will simply say, "Hi. We have no records on you."
If you have a friends' IP address, you can check to see if it knows any of their downloads. It even offers a widget for your website, blog or Facebook page that you can use to annoy your friends.
You can search any IP address, torrent hash or filename for results, or you can snoop on randomly-presented IP addresses that are in the database.
This won't go down too well for heavy torrent users who would rather keep their activities private, but it does raise questions about how easy it is to track this kind of information across BitTorrent trackers and other sources.