How does Ender beat Beard at the war desk

In school, the boys go through hours of classes, watch videos of the past invasions, and play in the game room--"what they lived for." Ender tends to go to the harder games to study tactics. After watching one game, Ender asks to play against one of the older boys, and Ender soon learns how to beat him. By the third game, Ender wins "quickly and efficiently." Some of the others start to use his tactics.