Board Thread:Naruto Discussions/@comment-28568301-20180910173149/@comment-1512913-20180910220932

Well, the thing is, we are not even 100% sure if there is such a thing as Yin-Yang Release, but in all likelihood, there is. Reasons: It has been called 2 different ways, both ways either refer to the same thing or 2 different things. In case they refer to 2 different things, then In'yōton most likely means 'Yin and Yang Release' (as in separately, not combined together) while Onmyōton probably means Yin-Yang Release as in combination of both natures. In case both terms refer to the same thing, we are screwed, because then we have no idea which instances refer to Yin and Yang Releases and which to Yin-Yang Release and as stated, if there is even a thing such as Yin-Yang Release.

But Sakura for example can use both Yin Release and Yang Release, yet there is no indication that she has Sage of Six Paths' power or that she is on that kind of level. There's more characters that can use both Yin Release and Yang Release, so while impressive, it doesn't seem too special, such that it would require Hagoromo's powers to achieve.

Therefore there must be this Yin-Yang Release because it was stated that only those with his powers can use it :D

But to actually answer your question, Izanagi is messy. Originally it was indeed implied that powers of 'both Uchiha and Senju' are required to use it, Uchiha predominantly having inherited Yin from Hagoromo and Senju Yang. I will use her as an example again, but Sakura can use both Yin and Yang, yet she doesn't have powers of both Uchiha and Senju now, does she? Therefore it was most likely contextually meant as Yin and Yang merged together into Yin-Yang, Hagoromo's power. Yet, later we are told another history lesson by Itachi and shown the Uchiha abuse and spam Izanagi. But this has 2 implications:
 * all those dozen Uchiha who used Izanagi in the past, stole some Senju flesh and put that in their bodies
 * and all of them technically had Sage of Six Paths'/Hagoromo's power (ain't that rare now, is it?)

That's total nonsense though. So we agreed that most likely Izanagi requires BOTH Yin Release and Yang Release, but it's not actually a Yin-Yang Release. But if it were so simple, Sakura would be able to use Izanagi... But the Sharingan is necessary of course, the main ingredient so to speak, besides Yin and Yang Releases.

This still doesn't explain though, how come so many Uchiha could use Yang Release. Sure, they could have learned it, but so many? Strange indeed, considering 4th databook states that they predominantly have Yin Release affinity. So let's just assume that many Uchiha decided to learn Yang Release, just so they could use this super hax powa' called Izanagi.... or they indeed put that sweet Senju nectar into their bodies (whichever you rather visualize) ;D