lol xD... I think it is some kind of counter to Amaterasu... or some sort of gengutsu, not a counter for sharingan, coz if I remember well, sasuke couldnt read naruto's movements when he was covered by Kyuubi's Cloak.
I think Itachi gives him something to block MS attacks cuz he wants to be sure that konoha will be protectedITS OBVIOUS
Itachi cares for Naruto and doesn't want him to have to kill his best friend....
BUT if Sasuke attacks Konoha (like he is now) the power is Naruto's ability to kill Sasuke!