It’s really easy for a law firm in Germany to find out who the IP belonged to, if they have proof that the IP infringed on their copyrighted media.
The law firm looks at torrents and downloads a bit. With the IP, time and media name they can send a cease and desist letter with a fine of hundreds to thousands of euro. Ignoring the letters is not possible.
This is possible because the law firm has contracts with many big copyright holders (Disney, …).
But most of the time the fine is too high, so it’s possible to pay half by getting a lawyer. Basically the copyright holder overestimate how much damages they can get for the distribution of copyrighted material. If I understand it correctly. IANAL.
It’s simple to avoid by binding the torrent client to the network interface of a VPN, but not everyone knows that.