Describe the Republican sniper's ruse to kill the other sniper.

From the text:

Taking off his cap, he placed it over the muzzle of his rifle. Then he pushed the rifle slowly upward over the parapet, until the cap was visible from the opposite side of the street. Almost immediately there was a report, and a bullet pierced the center of the cap. The sniper slanted the rifle forward. The cap clipped down into the street. Then catching the rifle in the middle, the sniper dropped his left hand over the roof and let it hang, lifelessly. After a few moments he let the rifle drop to the street. Then he sank to the roof, dragging his hand with him.