How did hassan make assef and his friends leave amir and him alone

Amir stands in for Amir to be the one persecuted while Amir runs away. Amir transports us back to the moment when he hid in the alley, watching Assef and his friends seizing Hassan. He remembers the blue kite and Hassan's pants lying on the ground. Assef told both his friends to rape Hassan, but they refused. They consented to hold Hassan down while Assef raped him. Amir saw "the look of the lamb," the look of defeat, on Hassan's face.