You are all correct that these are not downdraft stoves. But you are incorrect that they are not gassifiers, and that they are some kind of urban legend. Whether the technology is best put to use for backpacking is another question.
Technically the Bushbuddy and its kin are inverted-downdraft-gassifiers. Gassification happens when you force air downward through a hopper toward a burn chamber. When the combustible material gets close to the combustion, it begins to smolder, i.e. gassify. What the forced air does is force this smoke downward and through the hot coals.
for a demonstration of how it works in a home heater.
Since this smoldering uses up the available oxygen as CO and H2 (et al.) are produced, more 0^2 must be introduced in order for the gases ignite. The ignition of these gases is secondary combustion.
That is what we are seeing as the flames exit the top holes of the Bushbuddy. Plug those upper holes and you would have a smudge pot. Contrary to what appears to be the popular wisdom, it is not the preheating of the secondary air that is most important (though it is not insignificant); it is the draft of fresh oxygen.
Inverted down-draft gassifiers are also known as top-lit-updraft-gassifiers, and have quite a following among third-world development workers, with improvements in efficiency over an open fire by three to four times. See the discussions at http://www.repp.org/discussiongroups/resources/stoves/
These tinkerers are not just interested in efficiency, but in smoke and the other dangerous chemicals given off by non-complete combustion (ever gotten a headache from wood smoke? That's carbon-monoxide poisoning.)
see also http://www.woodgas.com/
Another thing that is missed is that true "top-lit-updraft-gassifiers" must be lit from the top, like Garlington describes. The Bushbuddy can be a pure t-lud stove if it is treated like one, but it is typically a kind of hybrid which works on the same principles. If anyone fiddles around, they will find that the _cleanest_ burn overall will result from starting the fire with some alcohol soaked pieces of wood on top.
Perhaps the most important thing the tlud burners do is regulate primary and secondary air supply; hence regulating primary and secondary combustion. I found the Garlington burner most unsatisfactory. I know now that it is because there was too much primary air feeding the "smolder" stage, and not enough secondary air feeding the "combustion" stage. The Bushbuddy has this carefully regulated---at about four times more secondary air than primary air (measure the intake below the ash pan, which is very hard to see, and compare it to the combined diameter of the upper burn chamber holes). This keeps the smoldering happening at a good enough clip to boil our water, but not so fast as to provide more smoke than the oxygen in the secondary air can combust. Hence the "clean as a candle."
By the way, every kind of wood or biomass fire, even an open one, is a kind of "gassifier," because combustion always happens in two stages: gassification of volatile organic compounds (i.e. smoke), then combustion of these gasses (flames). A well-built teepee fire will be smokeless---but notice how high the flames climb. They climb in search of more oxygen. What all kinds of stoves do is concentrate the heat and regulate oxygen to create more complete combustion and more efficient heat-transfer.
Good technique and patience to create dry wood and find a sheltered spot can make any fire at least efficient enough for backpacking. But who wants to spend that much time (in our precious time in the wilderness, anyway: I like wasting days with fire while at home) fiddling with fire?
And beyond all that, I love my Bushbuddy not because its the best possible solution, but because it is a well-thought, handsomely produced design. If this weren't what caused us to buy stuff in the first place, we'd all be sleeping dry and happy and rich under plastic tarps in dressed in thrift-store synthetics. I do that too, but even in my thriftyness, every once and a while a thing of beauty comes along whose concept and construction are good enough to arrest me.
Andrew "Cheapweight" Wilson