Captain Solo isn't fighting his smartest fight. He still believes he can end the fight quickly; that he'll just steamroll over Starfleet because he's bigger and in his estimation, stronger. This very line of thinking made him believe that it was a good idea to opt for a slow, but powerful overhand strike.

Imagine Han's surprise that the world is suddenly spinning.

Kirk's throw flips Han head over heels with him landing hard on his back. He still has air in his lungs, but one of the muscles in his back was strained by unexpected movement. His body arches in pain. The very same spasmatic movement saves him from being stopped by about an inch.

Han throws a haymaker at the inside of the other man's stomping knee and quickly pushes himself back up to standing.