The way "DNS override" works is exactly how it sounds. If a LAN client makes a DNS request for a resource that you've configured a DNS override for, the DNS forwarder will reply with that IP address instead of the IP address the DNS server which has authority for that device would. As an example, if you had a web server inside your DMZ and it was called dmz.webserver.cx, there would be a DNS server somewhere with that machine's public IP address. If you were trying to access that device from inside your LAN, you would not want to try to connect to the public IP address of that device, that would be inefficient (and wouldn't work without using NAT reflection which is an ugly hack). Instead, you'd configure a DNS override so that your DNS forwarder would hand back the private IP address of that device and the connection would be made directly from LAN -> DMZ instead of LAN -> WAN -> DMZ. Glad you have it working, I hope this makes the rest clear.