There are several options to create submenu: multiple showmenus, droplets (showchildren, sitemap, etc), snippets, modules. On the itoklantenservicent
rum.nl site I used this snippet:
<?php
// Submenu region, only shown when submenu's are available
ob_start();
show_menu2(0, SM2_ROOT+1, SM2_CURR+2, SM2_ALL|SM2_CURRTREE);
$childmenu=ob_get_contents();
ob_end_clean();
if ($childmenu!="") {
?>
<div id="sidemenu">
<?php show_menu2(0, SM2_ROOT, SM2_MAX, SM2_CRUMB|SM2_CURRTREE,'<h2>[menu_title]</h2>','','','',false,false); ?>
<?php show_menu2(0,SM2_ROOT+1,SM2_CURR+2,SM2_ALL|SM2_CURRTREE); ?>
</div>
<?php } ?>