This is much less important in practice because the small wood sawdust balls under a microscope reveal plenty of air space between them. Now we can look at practical questions. Just keep drilling long enough and hard enough so that the burnt wood sawdust pile around the friction area gets big and heavy enough not to move as you keep drilling. I have obtained up to 5 coals at once all around the drilling area this way. What about the fire saw.
Fire thong works on exactly the same principle.
Once the crack is full, the thong rubs back and forth against the same wood powder and the coal ignites there. The fireplow technique is harder to understand and doesn't work everytime because it relies on brute strength to generate sufficient heat in the pile of wood dust to ignite a coal at a random spot in the pile. As the pile of wooddust at the bottom of the plow gets heavy enough and cannot move, the heat transfer from the friction area ignites a coal on the bottom edge of the pile, which usually manifests itself as a coal on top of the pile when it is accidentally hit by the grinding stick. The principles are not respected accurately in this technique, and probably a random crack between the dust pile and the plow board must permit the formation of the coal. It will most probably look like an upside down fire saw.