It was the light from a sword skill. But there weren’t any in the Aincrad style that could hit while his right shoulder and back were open to me.

Even with my eyes opened wide with fright, I activated «Slant», no longer able to stop my sword.

Eugeo’s body savagely spun anticlockwise a moment later. A horizontal slash approached from the left, leaving a red light in its track.
This sword skill… is a one-hit technique for two-handed swords, «Back Rush». A countering technique to spin around when an opponent had your back.

But I had never taught Eugeo such a technique.

(Alicization Uniting, 25)