How is nosotroshq.com creating the navigation effect when you mouse over a nav item?

Any JQUERY experts know how nosotroshq.com is creating the effect in their top nav? For example, when you mouse over "ABOUT US" it slowly animates开发者_开发百科 down. Any ideas?

Here i have isolated the menu code! ;-)

http://jsbin.com/odabi/edit

Looking at the source, there's actually a second invisible DIV for each button that is offset vertically by one or two pixels and holds the mouseover state image... That div is faded in and out with hover events.