Instead of authenticating from the system to be backed to the back up NAS, the reverse method is more secure because the back up server is never exposed to the system to be backed up.
SSH is a very suitable method to do this because the transmission is encrypted. For the reverse back up method to work, some form of user privilege elevation is also needed, this can be done using sudo.
This method requires a connection to start from the NAS. It automatically implies that the NAS cannot be an appliance, but rather a server that permits customized scripts.