Here is a video demo of how it works:
As you can see, for the most part, this is structured data and doesn't appear to be data from unstructured parts of the web. In any event, this is a really nice and easy way to narrow the results down. Elisabeth Osmeloski at Search Engine Land has a pretty large write up on the new feature.
The main issue for me is that it requires Silverlight to run, which I don't have on my main machine.