How to get the primary menu item and child menu item effect?

Hi, in this "after" example picture (http://www.chapterthree.com/files/menutrails_after.jpg) there is the effect that when a "primary" menu item is selected, sub-menu items appear below (eg, 'Forum' selected and then 'Categories' and 'Discussions' items show). I was wondering how this is done? At the moment I can't figure out how sub-menu items appear below a parent item when that item is active! I realize I can add parent and child menu items to a menu, but it looks like (from the example picture above) there are two blocks, one for the primary menu items and another for the sub-menu (child) terms. Can anyone point me in the right direction how this is done?

Thanks.