How to create a drop down menu like in twenty eleven theme?

Question

I really like the drop down menu solution of twenty eleven theme:

http://twentyelevendemo.wordpress.com/

and would like to create a similar one but I cannot figure how it works, or is it based on some existing code for dropdown menus?

Is there any source code that can be used, or can someone explain the principle behind it?

I know internet is full of both js and non-js solutions, I've even made some myself, but IMO nothing comes near this menu, so help is appreciated:)