Experienced racing engine builders say that swirl polishing the backside of valve heads relieves stress and lowers the chance of valve failure. They would probably know. Racers know the most since they have tested their methods, failed many times . . . and usually know what works and what doesn't work. They spent the money, and us "hot street riders" can reap the benefits of their findings. That gray line is the lapped valve seat. Usually intake valves can have a narrower seat width (to get the fuel and air in a bit faster). Exhaust valves like a wider seat width . . . Why? The engineers say, since exhaust valves run hotter, the time the valve is closed (touching the seat of the head) all that heat can be transferred from the valve itself, back to the cylinder head - creating a "cooler running" exhaust valve - ensuring a longer life of the valve. That wide seat carries away the heat. Swirl polishing improves air flow, and I know the carbon doesn't stick as easy either . . . in my experiences taking apart cylinder heads.