“—SAINT DESTROYER!!”

The spear in Itsuwa’s hands exploded.

It was not a metaphor, nor was it a comparison, but rather, Itsuwa’s spear really became lightning. This sharp strike that flew in a straight line hit Acqua right in the abdomen, and without mercy, the bluish white purple electricity passed through his body, spurting out from behind, ripping the darkness of the night apart. This huge friction caused the cloth that had been used to hold the spear to let out black smoke.

[...]

Acqua’s left hand was on his hip.

He was not pressing against any wound, but at the place where his skin nearly got touched, he managed to use his hand to grab the tip of Itsuwa’s spear.

Most likely, before Itsuwa’s spear turned into lightning, Acqua forcefully grabbed the tip of the Friuli spear; and the reason why those special sparks had appeared was likely because he had done something to it before it fired, causing the effect to leak out slightly from the tip of the spear.

Toaru Majutsu no Index vol.16 ch.2 pt.8