What's the mechanism of controlling the navigation menu in Twentyeleven?

Hi!

I’m trying to add an action to the navigation menu. I wish that the secondary menu will appear gradually not so suddenly.

Before, I thought it would be CSS, so I checked the CSS with the help of Firebug. But I didn’t find. After that, I try to use jQuery to add an action. However, it seems to have a conflict with the original mechanism.

So I put a post in the support <<How-To and Troubleshooting>> asking where is the problem.

A guy told me the twenty eleven use CSS to control the navigation menu. So I went back to read the CSS sentences. But I still can’t understand how the CSS do to make appear the secondary menu. So anyone can tell me What’s the mechanism of controlling the navigation menu in Twentyeleven?

Thank you