What act of bravery does Amir witness of his father as they make their escape from Kabul?

At a checkpoint, the Afghani soldiers would have let the truck pass without issue, but one Russian soldier demanded a half hour with one of the refugees, a married woman. To Amir's dismay, Baba defended the woman, telling the Russian soldier that he had no shame and that he would "take a thousand bullets before [he] let this indecency take place." Amir felt ashamed that while Baba would give his life to save someone, he did nothing to save Hassan. The Russian soldier aimed the barrel of his gun at Baba's chest, but the shot that rang out did not kill him. It came from the gun of a more senior Russian soldier, who apologized for the first one, explaining that he was on drugs.