The power steering pressure switch is a device that stabilizes the idle speed of a car while the steering wheel is on full lock, either to the right or left. When the system exceeds power steering load limits, the switch sends a signal to the engine control module to compensate.
The purpose of the pressure switch is to keep the car from stalling when the power steering load limit is exceeded. When the engine control module receives the signal from the pressure switch, it increases the idle speed. The engine control module is an on-board computer that tells the car how to run.