Kai was trying to elbow the dude. Being restrained held him back just enough so he barely missed clipping his chin. I don't think being restrained had anything to do with being nailed by an elbow (again, not a fist) from the other guy, though. It was too fast for him to react.

The flying "sucker" punch that we keep seeing was also an elbow.

Not sure what happened to good ol' punches.