The integral power staring gearbox contains a conventional worm and sector gear, a hydraulic piston, and a flow direction valve. One type of integral power steering gearbox uses a spool valve. Another popular type has a rotary valve. Note that it uses small spool valve to control pressure entering the two power chambers on each side of the piston.
When the Steering wheel is turned to right, the pivot lever moves the spool valve so that pressure enters the right side chamber. This forces the power piston to the left and helps the sector shaft to turn for right. Pressure enters the opposite chamber when the steering wheel is turned to the left. A rotary valve type power steering gearbox has a small torsion bar to detect steering wheel turning direction and turning effort.
When steering wheel is turned the torsion bar twists and turns the rotary valve. The rotary valve then directs hydraulic pressure to correct side of the power piston.