Google Hack to See Results Pre-Penguin Update

There’s a crazy Google hack to see what the Google results were pre-penguin update.  I tried this on a few sites of mine that got penguin slapped and it works.  Not sure I understand why this works or the point of it…  if I had to guess I’d say that Google uses this to make manual reviews when a qualified site/person complains of their drop post penguin.
So what’s the hack?

Search for keyword and follow it with a negative sign and a URL.
Example, if I wanted to see the results pre-penguin for the terms CARS I would search for:

cars -ebay.com

It works and if you got penguin slapped you might not want to try this hack because it leaves you remembering the good ol’ days when your SEO work all paid off.