Menu - only active block open

Great module, nice work!

Question: how do I do this? With template tags or CSS?

On the home page: only toplevel items

Mainmenu 1 Mainmenu 2 > click Mainmenu 3 Mainmenu 4

On the next page:

Mainmenu 1

Mainmenu 2 (= active block is open) Submenu 1 Subsubmenu 1 Subsubmenu 2 Submenu 2

Submenu 3

Mainmenu 3 Mainmenu 4

So I want only the active mainmenu block open, the others closed.