it's funny how people go to these FAN sites for references. i know for a fact that i can answer a question about naruto just as well as the staff at narutofan.com can. but i'm not saying this to disrespect narutofan.com, i'm just saying that some of the information is just personal assumptions and theories.

anyway, for someone like sasuke, who is a natural born sharingan user, he needs to 'activate' sharingan in order to use it. when he is using it, whether it's copying by itself or sasuke is forcing it to, it uses up chakra. but for someone like kakashi, who had an uchiha eye implant, he needs to cover it up or it'll use up chakra because the eye will start copying things at random. the reason why it uses up chakra is because the eye isn't his and he can't turn it on and off like sasuke can.

brilliant!