Well, simply put, it crawls the web using a custom filter to find new torrent files as soon as they are made available. (or accepts them from users who upload their torrents directly)
Next it subjects them to a few algorithm-based tests to ensure that they are healthy and that if they contain adult content they are flagged appropriately.
Lastly, they are included into the index and they show up on the search results until they are removed by us or until they are dead (determined when nobody is left sharing it for 2 weeks).
The process is completely automated.