I am rendering the top-level elements of a Zend Navigation object in one place like this:
echo $this->navigation()->menu()->setMaxDepth(0); <
echo $this->navigation()->menu()->setMaxDepth(0);
If I got your question right, this is how I do it:
print $this->navigation()->menu()->renderMenu(null, array( 'minDepth' => 1, 'maxDepth' => 1, 'onlyActiveBranch' => true, 'renderParents' => false));
Renders only the submenu of currently active menu.