> I don’t recall how this worked

Unlinking files reduces their reference count. Once the reference count reaches zero the file is considered deleted and the space can be reclaimed. Every open file descriptor and hard link increases a file's reference count. So if you've got a file descriptor open on a file in a background task (daemon, nohup, screen, etc) and unlink it with rm or something the file's reference count will decrement but not go to zero. Only when that program closes the file descriptor will the reference count go to zero and the file actually be deleted.