Let's say we're preparing a new release of a certain site. New server(s) are set up to serve the new site using the same domain as the old version of the site. But since the site isn't released yet, the DNS records still point to the old server(s). Before we point the domain at the new server(s) we might need to have someone who's a non-developer to browse the site. That's where this image comes in.
This image provides a way of proxying the new site on the new server(s) to a browsable test (sub)domain without requiring someone to edit hosts files or using a DNS proxy. It's a really simple image, but it does the job.
PROXY_DOMAIN (the test domain you want the other site to appear at)
SITE_IP (the ip of the server hosting the site you want to proxy)
SITE_DOMAIN (the domain of the site you want to proxy)