Custom adaptation for my sub-menu script that allows you to force a specific sub-menu to be displayed. Full documentation here: http://christianvarga.com/how-to-get-submenu-items-from-a-wordpress-menu-based-on-parent-or-sibling/

functions.php

<?php

/* usage
 *
 * wp_nav_menu( array(
 *   'theme_location' => 'primary', 
 *   'sub_menu'       => true,
 *   'root_id'        => id_of_menu_item
 * ) );
 */

// add hook
add_filter( 'wp_nav_menu_objects', 'my_wp_nav_menu_objects_sub_menu', 10, 2 );

// filter_hook function to react on sub_menu flag
function my_wp_nav_menu_objects_sub_menu( $sorted_menu_items, $args ) {
  if ( isset( $args->sub_menu ) ) {
    if ( isset( $args->root_id ) ) {
      // force a specific sub-menu to display
      $root_id = $args->root_id;
    }
    else {
      $root_id = 0;
      
      // find the current menu item
      foreach ( $sorted_menu_items as $menu_item ) {
        if ( $menu_item->current ) {
          // set the root id based on whether the current menu item has a parent or not
          $root_id = ( $menu_item->menu_item_parent ) ? $menu_item->menu_item_parent : $menu_item->ID;
          break;
        }
      }
      
      // find the top level parent
      if ( ! isset( $args->direct_parent ) ) {
        $prev_root_id = $root_id;
        while ( $prev_root_id != 0 ) {
          foreach ( $sorted_menu_items as $menu_item ) {
            if ( $menu_item->ID == $prev_root_id ) {
              $prev_root_id = $menu_item->menu_item_parent;
              // don't set the root_id to 0 if we've reached the top of the menu
              if ( $prev_root_id != 0 ) $root_id = $menu_item->menu_item_parent;
              break;
            } 
          }
        }
      }
    }

    $menu_item_parents = array();
    foreach ( $sorted_menu_items as $key => $item ) {
      // init menu_item_parents
      if ( $item->ID == $root_id ) $menu_item_parents[] = $item->ID;

      if ( in_array( $item->menu_item_parent, $menu_item_parents ) ) {
        // part of sub-tree: keep!
        $menu_item_parents[] = $item->ID;
      } else if ( ! ( isset( $args->show_parent ) && in_array( $item->ID, $menu_item_parents ) ) ) {
        // not part of sub-tree: away with it!
        unset( $sorted_menu_items[$key] );
      }
    }
    
    return $sorted_menu_items;
  } else {
    return $sorted_menu_items;
  }
}