[jQuery] [Dropdown menu] Expand upwards!

[Pattan]

Hello.
I´m trying to make a tab menu with dropdown child menus.
Nothing strange with that, just a nested unordered list like this:
<ul>
   <li><a href="">Value1</a></li>
   <li><a href="">Value2</a>
      <ul>
         <li><a href="">Subvalue1</a></li>
      </ul>
   </li>
</ul>

Now to my problem/question: How to expand the submenu upwards?
Please see this "what I want" picture. 
http://api.mediatechdesign.se/jquery/tab_expand_menu.png

I have try with different types of animations, slideDown, but can´t
figured out.
          $(".tabs li").hover(
                function(){
                        $(this).children("ul").slideDown("fast");
                },
                function(){
                        $(this).children("ul").slideUp("fast");
                });