If you were trying to fake it you probably didn't manage to fool it 
But how and where exactly it picks up the IP behind the proxy's own IP, I don't know. I think it's one of those server variables from VIA or FORWARDED-FOR or whatever, but then again, I don't actually know. Woudl love to know it too, for my own proxy blocking script.
It works reasonably well to cut through anonymizer proxies in general. It seems to not work well for some institutional proxies where the Ip one step behind is an internal LAN ip or localhost which should be discarded.
But how and where exactly it picks up the IP behind the proxy's own IP, I don't know. I think it's one of those server variables from VIA or FORWARDED-FOR or whatever, but then again, I don't actually know. Woudl love to know it too, for my own proxy blocking script.
It works reasonably well to cut through anonymizer proxies in general. It seems to not work well for some institutional proxies where the Ip one step behind is an internal LAN ip or localhost which should be discarded.