No, that's right. (not that I'm a master). It's done to Heatsinks and CPUs that aren't perfectly flat. Take a piece of glass, tape a piece of sandpaper to it, rub HS or CPU on it until it's flat. That's the simple description. Only done in cases of extreme convexity or concavity, or by people who are looking to get that last few degrees of cooling out of their heatsink.