Something about the rinnegan and the Sage of 6 paths (any one remember him
)
I think that the rinnegan is rare, jiraya states it, and that one rinnegan
is to be possessed at all time.
Early in the manga Jiraya remembers Nagato
saying that when he saved Yahiko he faded out
or he was out of control, so it is possible
that the rinnegan is controlling Pein(Nagato)
And if the rinnegan is in control I'm assuming
that the sage of 6 paths is living through the
rinnegan and he only comes on stage when the yser of rinnegan
has deep emotions (Like the fox in naruto)
And when saw his friend hurt he got angry, nervous....
so then he blacked out and the rinnegan(The sage of 6 paths) took
over. Just my guess
(Part of the story was stolen from another person
but i agree with him)