Proxy ARP would have the raspberry pi provide its own MAC address as the MAC for the server IP. All the traffic from the clients to the backup server would have to pass through the Pi's network interface for the duration of the session.
With this ARP stand-in, it provides the server's MAC to the client so subsequent traffic to the server doesn't need to pass through the Pi.
With this ARP stand-in, it provides the server's MAC to the client so subsequent traffic to the server doesn't need to pass through the Pi.