Meats from joints such as lamb shanks and oxtails, as well as whole birds, are usually the main ingredient and are extremely flavorful when braised. That's because they contain a high proportion of connective tissue and fat, which breaks down during the cooking process and add flavor both to the meat and the juices. However, all this connective tissue and fat must be cooked very slowly, with just the right amount of moisture and pressure to disintegrate the tough tissue without overcooking and drying out the meat. Specifically, you are converting tough collagen in connective tissue to tender gelatin. This cannot be achieved with heat alone.
Pressure is necessary.
What happens in this slow process is that the meat is first seared, forcing the juices to concentrate in the center of the piece of meat. When properly packed and heated in a heavy pot, the braising begins causing the pressure to break down the collagen. When the pressure and temperature has reached its maximum at the center of the meat, steam pries open the meat fibers and the juices break through the seared surface, mingling with the simmering stock. The pressure and heat continue cooking the meat and slowly the juices and stock find their way back into the meat and make it deliciously tender. To maintain the proper temperature and pressure in braising, you must start with the right pot.