im trying to achieve something like this http://www.sohtanaka.com/web-design/css-sprites-wout-background-images/
if you scroll down you will notice a box on the left it will follow you when you are scrolling down i know that this is done with position fixed but how can i make this to scroll like that example only when the user is on a specific level of the page like when he gets to the middle of the page etc…