The Pex screencast was a bit mysterious without sound and comments. I've added a 'storyboard' to help you understand it:
http://research.microsoft.com/pex/screencast.aspx
Page rendered at Thursday, September 02, 2010 3:06:05 PM (Pacific Daylight Time, UTC-07:00)
Disclaimer The opinions expressed herein are my own personal opinions and do not represent my employer's view in anyway.