How to get the primary and secondary menu effect?

Hi, in the "after" example picture (http://www.chapterthree.com/files/menutrails_after.jpg) on the module page, 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). Can I ask how this is done? I'm still trying to get this module to work (can't understand why it's not working so far), but if anyone could point out to get the primary and sub-menu effect I'd really appreciate it. At the moment I can't figure out how sub-menu items appear below a parent item when that item is active!

Thanks.