“This set is about building a butterfly controlled by animated expressions. In this really easy set, we look at building a simple rig and driving its motion by easy expressions. All the driven elements are controlled by sliders and all of this without key framing!”
http://www.3dtotal.com/tutorial/softimage_xsi/rigging_and_animating_with_expressions_for_xsi/rigging_animating_with_expressions_xsi_01.php