Scrollable overlay
===

I'm using <http://jquerytools.org> overlay.  
I've found here way to stop the page behind jquery tools overlay from scrolling and it's great until my element in overlay is not higher then my screen.  
I know that it's possible because in <http://pinterest.com> it's working like I want. I was looking in their code, but was unable to understand how to make it.  
Any real (simple) example how to do this?