“The Blasting…Rod!! Is that what did it!?”
“It amplifies the magic’s power to 10 times what the target thinks it is"
[...]
He dodged based on the assumption that the length of the rapier would be 10 times the image he saw in his head, but a dark red hole was opened in his side.
“Gbh!?”
“When you assume the length will be extended 10 times, the Blasting Rod uses that as the basis for the 10 times amplification. Thus, the blade is extended 100 times.”

Toaru Majutsu no Index New Testament vol.18 ch.4 pt.2