How does Ender "control" the situation in the washroom

Bonxo wants revenge for his loss in the battle. Bonzo protects his honor and strips naked to even the playing field. Ender turns on the shower, letting the soap make him too slippery to hold. Ender pretends to ask Bonzo to stop, and then when Bonzo attacks him, he twists in Bonzo's grasp and shoves his head into Bonzo's nose.