It was a spinning back heel kick from Vitor Belfort delivered to the side of the head belonging to Luke Rockhold.
It was rather impressive.
And it was yet another first-round knockout for Belfort, who won in his native Brazil on Saturday night at UFC on FX 8.
Watch the knockout below. (Scroll to around the three-minute mark if you're not into the recap of the previous bouts on the main card, include Jacare Souza's first-round stoppage over Chris Camozzi.