Devil_Hex
Can it be used to kill them all easily? Use wind genjutsu on them all at the same time once he has an imprint of the counter for the juinjutsu, i Hope his Idea of hiruzen knowing about it turn out true because if the ancient Root in anbu(Yamato too) need to have a yamanaka check them for any reason they're dead meat Mc should do a seal with a similar intent for himself too(without the part of destroying the brain)
tbh, i think that if it was that easy to replicate the Yamanaka secret techinique the other villages and clans from warring states period would have done it already, learning it just by looking sometimes and doing some experiments with more or less 100 people with it being far from his focus over 3 years is quite strange, it feels too easy, not even the Uchiha or Hyuga stole it from them over the warring states period. Anyway i don't think it's very impactful Fujin knowing it, it would be really bad if that went to other villages, but i think it doesn't change much with Fujin.