hmm
i just thought of something... THERE is a chance that it does work on its own because it seemed to drain kakashi's chakra on its own... so im guessing that as soon as u turn on sharingan it starts doing stuff on its own, but u need chakra inorder for it to copy stuff on its own. sasuke fighting in the prelim exam proves it, because kakashi says not to use the sharingan, and he knew it drains his chakra.