Or to put it another way: If you use the "blade" side, you're actually cutting, but not removing, the plastic. If you use the back side, the squared-off shape of the blade actually acts like a tiny gouge or plane, and removes plastic, creating a groove that's the width of the blade's thickness. Doing it this way serves two purposes: One, you remove the plastic that's in the body's molded-in panel lines, giving you an accurately-scaled opening between the body and the panel you're removing (door, trunk, etc.), and two, you actually have more control using the back side of the blade... it won't tend to "jump the groove" as readily as if you were using the sharp side and cutting the plastic.