Gravity Feeds- - I will take a crack at it. It is what I smoke on. Stumps Smokers being the most common (I think).
Along side of the main cooking chamber you have chute that you fill up with charcoal, the charcoal burns at the bottom and as it burns up more drops down (thanks to gravity) to be burnt for heat. They are so well sealed up and insulated that there is not enough O2 for the whole tube of charcoal to go up in flames. Picture the smoker like Winemaster built, but without the burn grate under it but off to the side and then a chute about the grate running up the side of the smoker. The charcoal drops on to the grate as the other turns to ash and falls through the grate on to the pan. There is an opening from the grate to the cooking chamber to let the heat and smoke it.
In mine, I just put wood in the pan and let the falling ashes/embers get the wood smoking, some put chunks of wood in with the charcoal.
I am not sure that helped............if you search on gravity fed on google or something you should fine info.http://www.stumpssmokers.com/http://maplehollowbbq.com/
Looks like most of the detailed pics of them have come off the website. I know there were a bunch of copy cats being made. I suspect that is why the pics with the details of the "guts" were taken down.