The style of cooking meats over an open fire pit has been around since the days of the Peking Man. Many Southern barbecue lovers still consider pit smoking the best method for preparing barbecued meat. There's never been one specific design set up for a pit, but when I was a child you always began with a very large hole about six feet deep across and four feed deep. Then a layer of heat-resistant rocks was added. A heavy mesh screen was put down over the rocks, then a layer of hardwood such as hickory, oak, alder, or fruit wood. Once the fire got going and the white hardwood coals remained, the prepared meat (the whole, skinned animal) was lowered into the pit on a spit and the pitmasters, as the fire tenders were called, used their own carefully guarded secret techniques for getting moist, smoky, succulent results. A good pitmaster was a genius at controlling the low heat for the sixteen hours necessary for the meat. Nowadays, it's harder to find real outdoor pit barbecue. And pit-cooked no longer necessarily means that the meat has been lovingly tended and basted for hours at a stretch. Also, various states have very stringent laws governing open pits. Consequently, many fine barbecue restaurants and all commercial barbecue manufacturers use gas- or electric-fired equipment to control temperatures and conditions. A few landmark restaurants use real open pits. And, of course, each of the owners is proud of his or her accomplishment. Pork shoulders, butts and spareribs are the most popular meats to pit smoke. Once smoked, shoulders and butts are sold "pulled" and partially "pulled". Popularized and most prevelant in the South, "pulled" indicates that the cooked meat has been separated along the grain into shreds with forks or by hand before the sauce is added. Paritally "pulled" means that the meat has been partially separated, then cut into one-inch chunks or strands. Beef is also pit cooked, but requires a more moist heat than pork This can be simulated in home methods by tightly covering the cut of meat with a lid or aluminum foil.
From: A Passionate Cookbook by Jane Butel, "Finger Lickin' Rib Stickin' Great Tastin' Barbecue"