Joomla 1.5 Menu -> how top menu item stay active

Hi all, here is a message I posted on Joomla.ord forum without success, maybe people will be more inspired here :

saw a lot of posts on this topic, but no response fully satisfied me. I'm sure it's possible, because it's done on joomla.org website.
Let's take this example, on joomla.org website, there is a topmenu with following item: main, news, documentation, etc ...

when you click on news, for example, a new leftmenu is displayed, verticaly, with following item:
-submit your news
-general news&event
-Joomla! powered site
- ...

If you try to navigate on the left menu items, the "news" item in the topmenu is always active :
<li class="purple_active_menu active_menu">
<a href="http://www.joomla.org/content/blogcategory/0/33/">News</a>
</li>

The question is, how is it possible, and please do not mention extended menu module, there MUST be a way with mod_mainmenu, isn't it Huh

Thanks a lot, I'm sure this topic would interest a lot of people.