Here's a pretty competitive fight from Lumpinee. Kongsak is among the southpaw elite with a wicked left kick. It's no secret that left handed fighters throw their strongest shots from the left, and in Muay Thai, the left swing kick is one of those weapons that can be a game breaker. You'll notice that the shots from the left side line up perfectly against an orthodox opponent's body positioning.
Nong O is an accomplished fighter himself, though. At this level of competition, the southpaw opponent doesn't rattle him.