Testing undeployed site from EC2 instance?

Sites such as Web Performance Calendar » WebPagetest Private Instances in Five Minutes suggest you can use the WPT EC2 instance for “Testing of content not reachable from the public Internet.” Does anybody know of any good references on how this is best achieved? In particular, examples of how to configure the EC2 instances so they and WPT properly test a page running on a local network? Or does doing this actually require a locally setup WPT server?

It requires a locally setup WPT server (well, at least a test agent - the server can be anywhere).