Hit in back of head

His right hand finally moved. His fist swung wide out to the side, looking like it was
trying to aim for the side of Kamijou’s head. In boxing terms, it would be a hook. It was
a one-hit, one-kill pugilist skill targeting his temples, with a curving path parallel to the
ground.
With his foot pinned down, he couldn’t back away. His hazy awareness wasn’t able to
ascertain the attack or consider avoiding it. Kamijou instantly brought up his own hand
to protect his temples—
—And the fist caught the air.
(...?)
The pause didn’t even last a second, but Kamijou was puzzled. They were at such close
range their noses could touch—there was no way his fist could miss. Then why had he
made it miss?
Yes—he didn’t miss, he made it miss?
The answer came to him not a second later. Tsuchimikado’s fist, having passed by his
temple, went around and was going straight for the back of his head. As if wrapping his
hand around his neck to hug him.
The back of his head.
The vital spot judged by even karate and boxing to be against the rules, since it had a
chance of leaving lasting aftereffects.
A huge impact rang out.
“Gah—ba...ah!?”
The strength in Kamijou’s body all flew out of him at once from that one hit. His body
sank straight down. Tsuchimikado’s next blow passed overhead the fallen Kamijou, by
coincidence or otherwise.
However, Kamijou couldn’t turn that into a chance to attack.
Because of the so extremely savage, rule-breaking blow, he fell to the floor, unable even
to pick himself back up. His two arms trembled irregularly. His sense of balance was
shot, and he didn’t even know which direction he would stand up in. The strength left
his gut, and the contents of his stomach nearly came back up.

- Toaru Majutsu no Index Volume 4, Chapter 4