How does pg_basebackup manage to create a snapshot of the filesystem?

Hi,
I'm currently trying to understand how backups work. In the
documentation in section "25.2. File System Level Backup" it says that
filesystem level backups can only be made when the database if offline
yet pg_basebackup seems to do just that but works while the database is
online. Am I misunderstanding something here or does pg_basebackup use
some particular features of Postgres to accomplish this?

Regards,
  Dennis