Question

For example http://twitter.github.com/bootstrap/base-css.html try to scroll up. You will see the bar with

“Typography Code Tables Forms Buttons Icons by Glyphicons”

when you scroll pass the element it will add a class to make the element change. While you are scrolling back up it will remove the attribute to change the element again.

Edit: I found out this is call Sticky Element.

Answer 1

THat jQUery plugin can do what you want: http://imakewebthings.com/jquery-waypoints/

And here is an exemple taken from the URL:

someElements.waypoint(function(event, direction) {
   if (direction === 'down') {
      // do this on the way down
   }
   else {
      // do this on the way back up through the waypoint
   }
});

Cheers