Natural Draft is simply a created by a pressure differential. The hot air inside the flue is less dense then the ambient outside air. Difference in density creates the pressure that moves the air. So the pressure created is based on two things, temperature difference and air volume. More air in the flue will lead to more draw. Hotter air in the flue will lead to more draw.
The second one is the theory of the neapolitan vent. By routing the flue gases over the hottest part of the dome you heat them, creating a bigger temperature difference, more pressure, and more draw.
My oven draws great, as good as my old over sized straight up vent on start up, and much better once up to temperature, so it seems to support the claim so far, but I do have another theory. The neapolitan vent design is always said to increase fuel economy. A more complete combustion due to better air flow would cause this, but some would other things, like insulation. I am starting to form an opinion that neapolitan ovens may be more mass based(like most old world ovens) rather then insulation based like more modern ovens. I'm pretty positive some form of sand mixture is used over the dome. This would be thermal mass, and it is supported by the fact commercial neapolitan ovens get so hot on the outside you wouldn't want to touch them. My idea is that the neapolitan vent, and it's benefits actually come from it acting as a form of insulation in the dome that otherwise would have none. Heat moves from hot to cold, so putting an air cavity over the dome and filling it with hot flue gases would drastically reduce the flow of heat out the top of the oven, and essentially act as insulation. An oven built with all this mass over the dome and no cavity would burn more wood. Just an idea, but one I'm leaning toward.
As you can see, I can talk about this stuff for hours, and think about it non stop. Fagilla, if you have any questions not answered in my thread feel free to ask. What you see there is the result of literally thousands of hours of reading, research, thinking, problem solving, designing and improving. Much more then I could ever post.