moving nav bar

[gustcd26]

This may be a dumb and simple question but bare with me,
how can you get the effect of this nav bar http://www.swishzone.com/products/si.../mainmenu.html

Main question is if you click on about, it knows to move to the right, but if you click home, it knows to move to the left? (So how does it know which direction to go?)

Thanks!

[Ask The Geezer]

Open this and find out.