The Intelligence was able to control the Yeti and provide its motive power. Should the sphere be removed from the chest, it would move by itself to locate a deactivated Yeti and install itself into its chest. If the chest was blocked by something like a rock, the sphere would no longer think it is needed. The Great Intelligence was also able to take the form of a three sided pyramid of control spheres. (TV: The Abominable Snowmen, The Web of Fear, HOMEVID: Downtime)