A pressure cooker cooks food faster by

A pressure cooker works by trapping steam inside a sealed vessel, which significantly increases the internal pressure [3]. Boiling occurs when the vapor pressure of a liquid equals the surrounding atmospheric pressure. By increasing the ambient pressure, the water molecules require more kinetic energy to escape the liquid phase, thereby raising the boiling point of water above its standard 100°C. This elevation in boiling point allows the water and steam to reach higher temperatures without turning into gas prematurely [1]. Since the food is submerged in or surrounded by water and steam at these higher temperatures, the chemical reactions and heat transfer involved in cooking occur much faster than they would at normal atmospheric pressure. Conversely, at high altitudes where pressure is lower, the boiling point decreases, leading to longer cooking times.

Sources

1. [3] http://www.fsis.usda.gov/food-safety/safe-food-handling-and-preparation/food-safety-basics/high-altitude-cooking
2. [1] Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 3: Geological Time Scale The Evolution of The Earths Surface > 3. Geological Time Scale – The Evolution of The Earth's Surface > p. 43