If you look closely, you can see a tow strap pulling the wall out to fit the trusses. I always wondered how they put these up there and they do it by first hanging the truss upside-down on the walls, then using a long pole to push the tip of the truss upright. A guy at the top catches the truss and nails it in place along a long board. They run a string along the top to make sure the trusses all line up straight. You can see a few hanging upside-down in front of the man on the top plate.