invisible hit to the head

The staff’s tip made a sound when it came into contact against the marble pillar beside
her
(...?)
Kamijou frowned to himself at the strike that had been made outside her range, when—
A moment later, Kamijou’s vision had toppled ninety degrees to the side.
“Gah...! Agh?!”
By the time he’d realized something heavy and metal had hit him in the side of the head,
he was already crumpled on the floor. He desperately shook his unsteady head and
made sure he had a clear view; then Agnese, with the bottom of her twirling staff, hit the
marble floor with a thud.
Right as Kamijou, shuddering with fear, rolled along the floor, an impact struck it where
his head had just been a moment ago. The dull noise created depressions and fissures in
the floor like a hammer had slammed into it.

- Toaru Majutsu no Index Volume 7, Chapter 4 Part 4