One of the backup options on the ReadyNAS is using Rsync. Rsync is an open source utility that provides fast incremental file transfers between local or remote directories. To sync between two different systems one normally has to set up an Rsync server process on one or both machines. An alternative to this is to use an SSH tunnel between the two systems. However, this functionality is not present in the "Backup options" on the prosumer line of ReadyNAS products. Using this add-on enables the functionality for the non-business-grade ReadyNAS products (Ultra, Pro Pioneer, NVX Pioneer).
ReadyNAS Platform: Intel
ReadyNAS Firmware: RAIDiator 4.2.13 or newer
To ensure the add-on was installed properly, first head to the "Services" menu and click on "Installed Add-ons". Locate the entry for the add-on you just installed which may require scrolling down a bit. If it is listed proceed to check the functionality: Switch to "Backup" and select "Add a New Backup Job". From the drop down menu select "Remote: Rsync Server".
You will see a new option panel where you can select the "Tunnel Rsync over SSH" option. Also you can download the public SSH key of your ReadyNAS there. Depending on your selection this option is either available for the source or the destination of the backup but not for both. That's normal behaviour of the ReadyNAS and not a bug.