This is a feature that allows you to see the Exposé of an app that is in the background. You'll need a Trackpad.
In OS X 10.7, the four-finger swipe down toggles app Exposé for the app in the foreground. Except if you place the cursor on the icon of a running app in the Dock, in which case the four-finger down swipe toggles the app exposé of THAT app, not the application that is in the foreground.
If you swipe down again without selecting a window from the Exposé view, you are taken back to the app you were in before toggling Exposé.
[crarko adds: I'd love to test this, but my Lion system is still trying to finish installing the 10.7.3 update, which has already taken considerably longer than anticipated.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=2012020612504398