"But this could still be useful. Let's talk about a dueling sword called Whitting. When in battle, this sword will scatter when its owner is at a disadvantage, and amongst them, a miracle would occur…it's a spell that can accurately avoid damage and cause the sword to be sharper. When both are assembled, the sword's destructive power will increase exponentially."

[...]

He could nullify all fatal attacks, but he also had a sword spell that increases its destructive power every time he was attacked. Thus, when facing a fierce attack by the enemy, he could use a spell that has an infinite ceiling. This is different from Accelerator in that it's not the control of just one attack, but that the thing that’s worth noting is that once the power increases, he could maintain such destructive power when fighting the enemy in the future.

Toaru Majutsu no Index New Testament vol.4 main.31, Afterword