Steering wheel turned to left:

The valve piston (5) is adjusted to the right and the inlet groove (10) is open. The valve piston (4) is adjusted to the left and the inlet groove (9) is closed. Pressurized oil can now enter the left cylinder chamber. The oil in the right cylinder chamber is forced out and flows back to the oil container.

Fig 1: Recirculating Ball Power Steering Unit - Steering Wheel Turned To Left Position (Cross Sectional View)

Courtesy of BMW OF NORTH AMERICA, INC.