He could be moving with the blow to rob most of it with hit’s power. Punching Superman while he’s staying stock still would be like punching a brick wall with all your might.
You’d likely shatter your hand and tear open your knuckles, at the very least.
This is just Superman getting the point across without destroying the guy’s hands.
Comic drawing was going badly after such a long break so I thought Mikasa would be a good warmup (now if only the warmup hadn’t taken an entire day T_T)