That's clever, and I just understand after a while.
Now let's say that your website will show the ip of whoever visit it, in one of textbox. When you access it it shows your ip. When the proxy sever access it it shows the proxy sever's ip. When you access the proxy site, the proxy site will access your site, having their ip on one of the text box, then return the page with their ip to you.
The more advanced method is to encrypt the ip and put it hidden somewhere, on later for you to decrypt it, get the ip and black list them.
Now let's say that your website will show the ip of whoever visit it, in one of textbox. When you access it it shows your ip. When the proxy sever access it it shows the proxy sever's ip. When you access the proxy site, the proxy site will access your site, having their ip on one of the text box, then return the page with their ip to you.
The more advanced method is to encrypt the ip and put it hidden somewhere, on later for you to decrypt it, get the ip and black list them.