This is a fairly straightforward and naive attempt at detecting VPN or proxy usage by comparing latency times from
client -> server with latency times from server -> client. General theory is that a VPN or proxy will add latency in
one direction, but not the other.
To see it in action, you can play with it on my site, and there's also a better explanation of the theory and process behind it. Not that it's that difficult to figure out.