Menu with Dropdown

Hello everyone, I am creating a menu system very similar to the one found on http://www.macromedia.com. I think i’ve gotten it figured out except for the unloading of the subnav. If you roll over a nav. item, the subnav fades in. If you rollout of the nav/subnav the subnav fades away. How is this being done? please help.

Thanks!