[cinder-backup] incremental backup on nfs and ceph

good morning, i was trying cinder-backup to backup cinder volumes from nfs backend and ceph rbd backend. both my test show that i can do incremental cinder-backup, but i dont get how technically the cinder-backup could "understand" the new incremental blocks from previous backup? maybe on ceph it leverages ceph-diff? but on nfs backend, how it knows changed blocks? thank you