Michael, this is similar to facetting the limb, as Dean Torges show and speaks of.
I was taught this by a good friend, when I started building bows. His foundation was instrument building.
Certain cutting tools are used for flat surfaces, and others for radius surfaces. You bevel the edge down to the line, then take the center ridge of the limb down. This keeps you from "loosing your way" so to speak in the reduction process. Maintaining a good line down the limb for floor tillering!
Gary knows his stuff! I have his dvd, it's great, but I have a feeling there will be another!