left hand

In addition, his free hand was already on the next cross. Even if he let the first one be
blown away, Biagio’s attacks would continue. Blow for blow, the crosses were more
powerful than Kamijou’s fist. If Biagio tried to overwhelm him with numbers, Kamijou
would be pushed back for certain.
(I can’t get in close the normal way.)
At point-blank range, Kamijou turned his attention to his tightened fist.
(It’s not just this attack. I need to stop his entire flow of attacks...!!)
There was no time now that he thought of it, even if he tried to prepare something.
In the end, he had to bet it all on his one fist.
The cross in the palm held in front of him popped and began its expansion.
“Biagioooooooooooooo!!”
At the very same time, he sent his fist flying at the accessory.

Not his right—but his left.

Even an amateur could tell his nondominant hand’s strike was much weaker. Compared
to his right fist, which he always used to punch, its speed was overshadowed.
But his left fist was different in one other respect.

It didn’t have the Imagine Breaker’s power in it.
“!!”
The cross Biagio was gripping bounced a little from Kamijou’s left fist. With a soft sound,
the accessory in his hand had simply changed its direction slightly.
However...
The cross, facing in a different direction than Biagio had planned, expanded very quickly.
The tip of the cross that had been in his hand now stabbed his jaw.
“Ghbah?!”
Biagio’s body sprang straight up.
(This...little...He used my own attack...!!)
He could think, but he had no room to speak. The whole inside of his mouth stung with
pain.

- Toaru Majutsu no Index Volume 11, Chapter 5 Part 9