Board Thread:Naruto Discussions/@comment-25192437-20150109095203/@comment-34870248-20180302125025

Basically. When Hagoromo and Hamura fought Kaguya, both brothers possessed one half of her shared chakra, Hamura had yin, and Hagoromo had yang. When they sealed her with chibaku tensei, the ten-tails was split into 9 souls (9 tailed beasts), and sealed within Hagoromo. Thus, at that point his power was equivalant to Kaguya. When Indra and Ashura were born, the brothers possessed one half of Hagoromo's chakra. So, Indra had yin, and Ashura had yang. When they reincarnate, their chakra is passed on (now to Naruto and Sasuke.) So basically, their power is still there because Naruto and Sasuke has Indra and Ashura's power along with power from Hagoromo.