Uchiha brother Itachi’s never-ending flame technique, Amaterasu, sprung out of Sasuke like a deadly trap when he made eye contact with the supposed “Madara.” But how did Itachi implant Amaterasu in Sasuke?
Itachi was able to implant the Amaterasu in Sasuke by tapping his forehead before death. In that moment, Itachi transferred his ocular powers to Sasuke and laid the Amaterasu trap for those who may try to reveal secrets to his younger brother.
Amaterasu was transferred in order to act as a last line of defense against Madara if attempts an audience with Sasuke.
Plot Behind How Itachi Could Implant The Amaterasu
After “Madara” retrieves Itachi and Sasuke’s bodies from their deciding battle, he treats Sasuke’s wounds in preparation of telling him the full truth of Itachi; the answer as to why he betrayed the Uchiha Clan.
In Chapter #397, before Madara can begin his explanation, he makes eye contact with Sasuke and becomes engulfed in the Amaterasu’s never-ending black flames. He’s able to use his Rinnegan abilities to separate himself from the flames and return to Sasuke’s side unscathed.
The Amaterasu, which Itachi seems to have implanted in you. Leave it to Itachi to set a post-mortem trap. I can’t believe how well he plans ahead. In order to kill me, Itachi placed a jutsu on you… Or at least to keep me away from you.
Madara to Sasuke | Chapter #397
Madara asks Sasuke to remember a moment when this transfer could have occurred. Sasuke is shocked, recalling the moment when his older brother tapped his forehead before death. It was at this moment that Itachi implanted the Amaterasu within him.
Itachi must have done something to you before he died. At the very end… He transferred his own ocular powers to you.
Madara to Sasuke | Chapter #397
So that’s the answer. Itachi was able to implant Amaterasu in Sasuke by tapping his forehead before death. In that moment, he transferred over his ocular powers to Sasuke and laid the Amaterasu trap.