Geez.. seriously. That shot didn't look that bad (maybe it was the strongest short left hook ever thrown, lol) and it put him out cold. Not just rocked or flash ko'd, but lights out completely. His chin is made of cracked glass. He should be the only fighter allowed to wear headgear into a fight rofl.
That's a wicked little punch. I realize there are people on here saying it didn't look like much of a punch, but if you watch the tape, you can see the Petkovic cocks his entire body back for that shot. You can actually seem him lift the front foot and drop it into the left hook.
It's not a traditional, hip-and-shoulder-swinging left hook, but it's damn effective.