I usually burnish the backs of my bows, like crooket said, to keep splinters from lifting. I find it also helps with the finish, especially on red oak, to keep all that early wood from sticking up when staining and finishing. I have been wary of doing it to the belly, because for some reason I thought it might cause chrysals. You're saying it can increase the performance of a bow by burnishing the belly? I'm interested.