[Lan Wangji goes to where the chaos is -- he doesn't start it. which means that he usually won't be the first to strike in a fight, but he's always, always prepared to see one start.

when Six lunges out, Lan Wangji is already moving to block the swing, using the force of his own forearm to push Six's to the side and up so the movement will go wild and undirected instead of hitting his target. he'd normally try to stay out of Six's way by redirecting and subtly shifting out of the most straightforward line of attack, making him duck and weave and turn to keep up with the speed of his opponent, but Lan Wangji doesn't have that luxury here. the kappa is behind him, and he doubts Six will be distracted by fighting a defender one on one when he's made it clear that doing the job is his goal.

instead, he needs to be brutally direct in turn to create an opportunity for the kappa to get away. so he follows that warning strike with one of his own -- a swing with one fist at Six's stomach while he's close and his weight is shifted forward. it's an obvious enough move that Six will probably see it coming, and brutally quick enough to show Lan Wangji is serious about standing his ground.]