I found this interesting theory
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in NB that actually made sense to me. Give it a try. Well here are the lines of where his theory starts...
"Sasuke {Calm and ignoring manner} : That day at VOTE you used LIMBO and seperated your shadow at the last moment. Only those who have awaken two abilties in both MS can awaken the "Third Power" - SUSANO'O. Isn't that right? Limbo was your second ability. Though even when it was your eyes' second ability it consumed an immense amount of Physical Chakra for even one time use,thus making it an unusable battle type ability and putting it in the category of Escape Jutsu. That handicap got removed the moment you implanted Hashirama's Cell and acquired his/Ashura's immense physical chakra and awaken the Rinnegan,but even then you can use it for 2 times max I assume,though now,thanks to Jubi even that handicap is totally nullified.
Kakashi : But that only explains his survival at VOTE Sasuke,not immortality.
Sasuke {Smiles calmly} : The technique divides your very soul into two and thus making you immortal,or so it seems. As long as one soul remains other can't be killed. That and of-course the fact that you are now Jubi's Jinchuriki which helps in your quick body regeneration. Though that's where Mine and Naruto's ability comes in. We used the power of SO6P to not just cut you,but to cut you in a way that you can't rejoin yourself. Also in a way to decrease your power. Simply put,all your parts are now just 1/7th part of power of your original. Meaning you all are 7 times weaker than your original self."
