How exactly does a pressure cooker work?

The principle of pressure cooking is really quite simple. Because a pressure cooker is airtight, pressure builds up inside the pressure cooker as the liquid inside comes to a boil. The resulting trapped steam causes the internal temperature to rise beyond what it would be capable of doing under normal room pressure. Then, simply, food cooking at a higher temperature and under pressure cooks faster.

At sea level (where the normal atmospheric pressure is 14.7 pounds per square inch), the boiling point of water is 212° F, the highest temperature which can be reached by water at that elevation. That boiling point is raised 38° F to 250° F under the 15 additional pounds of pressure that can be obtained in a pressure cooker. Foods cooked hotter cook faster.

A secondary benefit of the increased pressure is that it softens the fibers in foods, tenderizing even the toughest of meats and beans. The increased internal temperature and the assault on the fibrous nature of foods cooked under pressure combine to decrease by two-thirds or more of the normal cooking times.