Pork shoulder and beef brisket, which are full of connective tissue, can come out tough unless they're cooked using a low-and-slow method, which breaks down the tissue and leaves the meat gorgeously tender (think pulled pork or barbecued brisket). But cooks who braise, slow roast, or smoke these meats at low temperatures for long periods of time experience a cooking conundrum related to reaching that goal.
The trick to achieving tenderness in these cuts is an internal temperature of near 200 degrees Fahrenheit, the point at which the meat is sufficiently done for pleasurable eating. Once it hits about 150 degrees, however, the internal temperature doesn't rise for a long time—sometimes hours. Barbecuers call this phase "the stall," which is caused by a process known as evaporative cooling. (Here's an article that explains the science of it all, complete with graphs.)
When you hit the stall, you can take it in stride and "have a beer and wait it out," says scubadoo97. Once the meat reaches 180 degrees or so, the temperature will rise quickly.
But there are time-saving solutions to this quandary. If you are using an oven, cover the pan with foil. If you are using a smoker, wrap the meat once it has developed a crust and smoky flavor. These strategies will shorten the overall cooking time, says scubadoo97, who also notes that understanding the process "doesn't really change what we have been doing for years but does make you more conscious of what's really happening so you have better control [of] the environment."