The hidden panel in the nimbus template has been implemented in a slightly different way to previous Joomla Bamboo themes. This time its a little subtler and a little bit slicker. The panel itself is triggered by the button on the right edge of the template and sits on top of the main content body.
The transition effect is a very funky fade effect using Jquery which means that it looks like the panel morphs into the main content area and morphs out again once the panel is deactivated. The active state of the panel is indicated by the panel button fading to around half opacity - it toggles back to full opacity once the panel becomes deactivated.
Setting up the hidden panel is as easy as publishing a module (any module) to the cpanel position. You can have multiple modules published here or just one.