I reread the thread now that we have a bit more information , I can correct some things.
Firstly , Hagoromo's brother is now a mysterious character named Hamura:
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Secondly , Hagoromo was probably Yang (I thought he was Yin) , his brother was probably Yin:
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That changes a lot for my theory , the guy that seems to have dispersed his Yang descendance (Kaguya clan , Juugo clan , Uzumaki clan , Kin and Gin ...) around the world is Hagoromo.
He found a way to connect his spirit to another human spirit via physical contact , he called that ninshuu.
-If a chakra network is needed for this connection between 2 human spirits , he may have found a way to give chakra networks to other humans to connect spirits , Hamura's Byakugans could have been a great help to do that.
He could have dispatched that ability during his trip , dispatching Yang dominant chakra networks around the world.
That would explain all the Yang dominant clans that are dispersed around the world.
Later Hagoromo got the Yin power of his brother and was able to create his 2 sons Ashura and Indra.
Can't wait for part 3!