I would guess that they do it similarly to private trackers. Each user gets a unique and secret ID which is transmitted when announcing. The tracker then decides if the user is allowed to get the swarm details.
I just purchased the new Thom Yorke album [0] via a bundle and it felt like a regular torrent. Very quick to download etc.
[0] https://bundles.bittorrent.com/bundles/d0b4beba8efc4b46f6dba...
udp://tracker.publicbt.com:80/announce
http://tracker.bittorrentbundle.net:3000/3pr4s5y7l9nh0mdwcqyh3a9k6n7d0kta/announce
http://tracker.bundles.bittorrent.com/tracker/{a whole lot of hex}