The thing on top of a pressure cooker that helps regulate and release pressure is called the "pressure regulator" or "pressure valve." This is a crucial component that ensures the pressure inside the cooker does not exceed safe levels.
Modern pressure cookers typically have a pressure regulator that consists of a weighted valve or a spring-loaded valve. When the pressure inside the cooker reaches the desired level, the pressure regulator begins to release excess steam to maintain a constant pressure during the cooking process. This prevents the pressure from building up to dangerous levels and helps ensure the safety of the pressure cooker.
It's essential to follow the instructions provided by the manufacturer regarding the proper use and maintenance of the pressure regulator to ensure the safe and effective operation of the pressure cooker.