"Hide"? That is correct, dark will auto 'hide' objects for you if they are outside the cameras view frustum (note I said OBJECTS, dark will NOT auto hide LIMBS, only OBJECTS). It hides objects pretty fast (by maintaining its own internal quadtree), so it works fine in most cases.
However, if you want to EXCLUDE a object (which will almost always give you a performance boost), you need to do that manually. That menas YOU need to determine if a object is in or out of view. You can do this a couple ways - the fastest but most complex would involve maintaing your own quadtree, the simplest would be using the dbObjectVisible() command (which is kinda slow).
All you need is zeal