Both the increased temperature and the high pressure cause the food to cook at double or even triple the normal rate. Collagen in the meat also melts much more quickly into gelatin, which is a process that normally takes several hours.
One of the risks of cooking meat in a pressure cooker is winding up with meat that's as dry as cardboard since much of the moisture gets squeeze out during cooking. Be sure to use cuts of meat that are high in fat or collagen when cooking in a pressure cooker, which keep the meat moist and tender.
What do you cook in your pressure cooker?