Every private
tracker will know what you download/upload. When you register,you're given a PID (Personal ID) - some call it Pass key/UID.
This PID is found in profile/settings.
When you download a torrent,your PID gets included in that torrent so whatever you download/upload,the site knows it.
And each user has unique PID,so never reveal your PID to anyone. Tried to answer simply but if you've got questions,I would gladly help.