Pressure cooking is a method of cooking in a sealed vessel that does not permit air or liquids to escape below a preset pressure. Because water’s boiling point increases as the pressure increases, the pressure built up inside the cooker allows the liquid in the pot to rise to a temperature higher than 100 °C (212 °F) before boiling.
Since pressure cooking depends on the pressure of steam, the process cannot easily be used for methods of cooking that produce fairly little steam, such as roasting, pan-frying or deep-frying. Most pressure cookers sold in the U.S. have an internal pressure setting of 15 psi (1 bar), the standard determined by the USDA in 1917[citation needed]. At this pressure boost relative to sea-level atmospheric pressure, water boils at 125 °C (257 °F). The higher temperature causes the food to cook faster; e.g., cooking times can be reduced by 70 percent. For example, shredded cabbage is cooked in one minute, fresh green beans in three minutes, small to medium-sized potatoes cook in five minutes (depending on thickness and type), and a whole “roast” chicken takes only twenty minutes. It is often used to simulate the effects of long braising or simmering in shorter periods of time.
http://en.wikipedia.org/wiki/Pressure_cooker