pressure= Force / Area
the ring, especially if its big, will have a much smaller area than your knuckles, you generally land with several in a punch. Force is unaffected. but since there is less area where the same force is applied, it will hurt alot more. Additionally, it will hurt your finger more, so it would be most productive to have the outside of the ring sharper than the part thats touching your flesh. It doesn't have to be pointy, it could just be rounded. But if its like a standard wedding band it'll add almost as much damage to your finger as it would be to the point of impact (almost as much, because a wedding band is still somehwat rounded).