Rsync allows synchronisation or backup of files in one direction. i.e. you can specify which version of each file you want mirrored to both locations.
To enable the RSync server on FreeNAS:
Go to: Services -> Rsync
In the Rsync Server Settings tab:
Map to user Guest; TCP 873
In the RSync Server Modules tab:
Click on the little + symbol near the bottom right of the table.
Name: FreeNAS_rsync_server_module_name (substitute for your own alternative)
Comment: your own identifier
Path: browse to server backup location
Others: leave blank or accept defaults
Click Save near the bottom
You then need to set up RSync client on the networked PC you will be backing up.