Animated scrolling pages with effects?

The 2 sites below are animated scrolling pages which I know how to do, What I am looking for is how do they make the effect like the pages are sliding independent "like an overlaid effect" of each other If anyone can show me how they get that effect or if there are any demo or examples to help me figure it out


http://www.sketchtravel.com/
http://www.foofighters.com/us/discography

thanks for your help

-rob