But they let go of him. And as soon as they did, Ender kicked out high and hard, caching Stilson square in the breastbone. He dropped. It took Ender by surprise -- he hadn't thought to put Stilson on the ground with one kick. It didn't occur to him that Stilson didn't take a fight like this seriously, that he wasn't prepared for a truly desperate blow.-- From Ender's Game, by Orson Scott Card.
For a moment, the others backed away and Stilson lay motionless. They were all wondering if he was dead. Ender, however, was trying to figure out a way to forestall vengeance. To keep them from taking him in a pack tomorrow. I have to win this now, and for all time, or I'll fight it every day and it will get worse and worse.
Ender knew the unspoken rules of manly warfare, even though he was only six. It was forbidden to strike the opponent who lay helpless on the ground, only an animal would do that.
So Ender walked to Stilson's supine body and kicked him again, viciously, in the ribs. Stilson groaned and rolled away from him. Ender walked around him and kicked him again, in the crotch. Stilson could not make a sound; he only doubled up and tears streamed out of his eyes.
Then Ender looked at the others coldly. "You might be having some idea of ganging up on me. You could probably beat me up pretty bad. But just remember what I do to people who try to hurt me. From then on you'd be wondering when I'd get you, and how bad it would be." He kicked Stilson in the face. Blood from his nose spattered the ground.