vol 2
Once Tosen had received an explanation of the Full-bringers’ existence, he still could not understand how that was connected to the Soul Society’s sin. Expressing that Tosen’s doubt was natural, Aizen began to speak of what had been “before.” “Well, why do you think the unborn children’s mothers are attacked in the first place?” “Huh? So it’s not by coincidence?” Aizen quietly confirmed that, and Tosen the Soul Reaper was surprised. “You know that I was shaving away the souls of the people in the Rukongai and offering them to the incomplete Hogyoku, don’t you?” “Yes.” “With some of them, though they did not satisfy the Hogyoku, it exhibited a very strong reaction. When I researched further I found it reacted to a certain girl whose konpaku I had taken. To be more accurate, it seemed to react to the ‘nail’ that was intermingling in her konpaku.” Aizen had continued to research the Hogyoku for a way to bring down the royal palace and put himself in heaven. No matter how many hundreds of souls he fed it, the Hogyoku showed no signs of becoming complete. That was why Aizen wanted the other Hogyoku that Kisuke Urahara had created, but in the process of his research, he had by coincidence noticed the existence of the “nails” and the special abilities of Fullbringers.
When you speak of ‘nails’…is that a metaphor?” “No. I mean a nail.” At that point, Aizen smiled boldly and gave an explanation that was beyond what Tosen could have imagined. “A part that had been carved away in the distant past by the ancestors of the Soul Reapers—a part of the Reio’s body.”
So it was possible that defensive instincts would bloom in young souls that had been intermixed with peculiar spiritual pressure, if they were being attacked by highly perceptive Hollows. Or in the opposite case, that people like Orihime Inoue and Chad, who had passed down the Reio’s factor generation after generation, would have their defenses bloom alongside Hollow attacks.
thats what I found, probably there is more but I can’t search again right now. Basically hollow triggers the soul king fragment present inside am human that will develop an ability.