“Academy City esper powers are controlled using high level calculation ability. Even if you don’t normally think of it that way, you are still carrying out complex calculations without knowing it. Throwing that out of order can be quite useful. There’s no harm in starting with that, don’t you think?”
[…]
“Now then, will you lose the conditions needed for your reflection? Or will you bring out those much, much more insane wings? However this goes, I’m looking forward to it. I do hope you at least make this worthy of testing out my ability!”
[…]
Accelerator was unable to evade in time and his shaken thoughts were unable to fully implement his reflection. One of the spears tore a shallow wound along the top of his shoulder. Accelerator’s body was thrown through the air in a tailspin. He just barely managed to avoid collapsing to the ground and instead landed on the tips of his toes. Only then did he comprehend the terrible scene before him.
[…]
He tore at Accelerator with his words.
He dug into him.
He thought he had cut off all forms of airborne vibrations, but whether he liked it or not, that voice that was filled with true malice tugged at him, pried open his mind, forcefully slipped inside, and stabbed into his heart. Those thorns he could not pull out robbed him of his composure bit by bit. The equations that controlled his great power fell into disarray.
[…]
He was so filled with anger that he almost lost his balance. And only when he was truly cornered there did he finally realize something. Not only was he unable to control his power, but he was not even properly carrying out the calculations; he barely even noticed anymore that he needed to stand and walk on his own two feet thanks to his injured brain.

Toaru Majutsu no Index New Testament vol.6 ch.7 pt.4,6