How can I use ZC category navigation style on another site?

I've just done two Zen Carts and like the way the category box navigation works: when you click on a main category the subcategories become visible as long as you click links within that category. Subcategories stay visible until you click on a different main category. http://avianoutfitters.com

Can someone tell me how that is set up? I'd like to use that effect in a regular web site but am not up to speed with CSS.

I've used the Firefox CSS tool to look thru the navigation and button styles but I'm not finding any clues.

One way solution
http://javascript.internet.com/navigation/expanding-menu.html

Thanks for the lead, I didn't even know what that menu style was called. I thought the Zen Cart menu was purely CSS, but just read that can't be done because of IE (what else). The one you suggested should work just fine!