When you see a swirl mark what you're actually seeing is the very sharp edge of a very fine scratch. This scratch is in the clear coat, so the edges of the scratch are acting like a prism and the light is being reflected and refracted back at you. You see a pretty bright white light, and contrasted against a dark underlying color coat they really stand out. That's why you see them more readily on black cars than you do on white, or on non metallic cars more so than on metallic paint - the metal flakes give enough "background noise" so as to mask the appearance of the swirls. The act of using an abrasive suspended in a liquid that is rubbed against the paint with a pad (which is really all you're doing when you buff the paint) will round off these sharp edges and minimize or eliminate the appearance of the swirls. Taken to a higher degree you will literally grind down the clear coat and effectively level the paint to the bottom of the scratch. Sounds scary, yes, but your typical swirl marks are so fine that removing takes hardly any clear off the paint. Unless you go overboard, or start wet sanding, or just getting super aggressive with a rotary buffer, a wool pad and a heavy compound.