In the Southern United States, barbecue doesn't refer to the activity with ground beef patties in the back yard, but to the careful, slow smoking of pork and beef until the meat is fall-off-the-bone tender and full of smokey flavor. The reverse-flow smoking method is a way of making Southern-style barbecue.
Cooking the meat slowly and evenly is the first tenet of good barbecue cooking. To keep the cooking slow, the heat is separate from the place where the meat is cooking. There will be a firebox attached to the smoker so the wood fire won't be too close to or -*+++++++++in the same space as the cooking meat.
Hot smoke does the work of cooking barbecue. Hickory smoke is one of the most popular, but other common woods include alder, applewood and maple for the characteristics of their smoke. Letting the meat be surrounded by smoke allows the smoke to soak into every pore and crevice and impart its flavor throughout.
The smoke needs to flow from the firebox over the meat and out of the cooker. If the smoke were to stay trapped in the cooker, it would eventually settle and leave a layer of soot on the barbecue; not appetizing. The flow of smoke takes it slowly away from the fire and allows it to rise through the area where the meat is cooking and finally out a chimney or vent.
Installing a heat plate under the meat but above the smoke entrance from the firebox causes the smoke to travel the entire length of the smoker before it can rises to touch the meat. Then the chimney is placed directly above where the smoke comes in from the firebox so the smoke has to travel all the way back across the length of the smoker, across the meat and out the chimney. This causes it to flow one way to get to the meat and then to reverse its flow to exit, passing through the meat in the process. Reverse-flow smokers have a much more stable cooking temperature and even heat distribution. (see Resources)