If you drag and release quickly from the horizontal menu, the menu will auto slide for a distance. It seems to slide more the faster you drag and release. Source ->appear.dk
This technique is called kinetic scrolling (you simulate kinetic energy). Since the dawn of the iphone this technique has been hyped, though it can be used on mobile devices with great benefit, I would refrain from using it inside a website (like all other sorts of scrolling)
However, here's an easy to use jquery plugin: Here you go
None of those jquery librarys listed above .. can compare to custom prototype code used in appear.dk The smoot
Overscroll didn't work well for me on the iPad. Here are some more options: