If you use encryption (I always change the settings from “prefer” to “require” encryption on every install), the ISPs literally can’t identify what you’re downloading.
So the IP enforcement companies send the ISP a letter saying “this IP was illegally downloading our stuff. We don’t actually have proof, but trust us and punish them.”
Big surprise, a ton of ISPs just ignore them.
Edit: to be clear, I’m only saying encryption prevents your ISP from seeing what you’re downloading. IP (intellectual property) enforcers who participate in the torrent are the ones who inform your ISP, but their letters to the ISPs have no teeth. Some ISPs care, but a lot just ignore the letters. You still definitely want to use a VPN for all public trackers.
That’s what I said though.
Your ISP can’t see what you’re downloading if you encrypt, but the IP (intellectual property) enforcers still can if they’re participating in the torrent. Then they find out which ISP that IP (address) belongs to and sends them a letter saying “we caught your subscriber downloading XYZ illegally.”
However, at least one US district court has ruled that just catching an IP address downloading a torrent illegally isn’t proof that any particular person illegally downloaded the IP (intellectual property). As a result, some ISPs simply ignore the letters the IP enforcers send, while some of the bigger ones count “strikes” against the subscriber with that IP address.