A "ruse" is a trick or decoy meant to mislead one's opponent. The titular sniper needed to trick his enemy in order to survive.

The sniper first took a risk by lighting a match so he could smoke a cigarette. This gave his enemy a target to shoot at. The enemy sniper also appears to have a good view of the roof, because the sniper is unable to reveal himself in any way without being shot at. This results in him being hit in the arm, which makes his position even more difficult. With a wounded arm, he can't use his rifle, which means that even if he was able to see his enemy, he wouldn't be able to shoot him first. Plus, the sniper knows that if he waits until morning, he may die of his injuries, or reinforcements may arrive, or the light may simply make it even easier to see him if he delays his escape. He is in a defensive position, but only an offensive position can win; however, to switch roles, he needs to trick the enemy sniper into giving up his advantage. 

The ruse is that the sniper puts his hat on the tip of his rifle, raising it above cover and allowing it to be shot, and then he pretends to be dead. He puts on a convincing show, and the other sniper believes it, standing up out of cover long and clearly enough to be shot. The connection between the ruse and the enemy being hit is that his ruse causes the enemy to lower his guard and reveal himself in order for the sniper to take his shot, thus switching their roles.