Steering wheel in neutral position:

Oil flows from the vane pump to the worm head, through inlet grooves (9) and (10) to radial grooves (11) and (12). From here, via connecting bores to the right and left cylinder chambers and via opened return flow grooves (13) and (14) back to the oil container. The valve is also illustrated in cross section.

Fig 1: Recirculating Ball Power Steering Unit - Steering Wheel In Neutral Position (Cross Sectional View)

Courtesy of BMW OF NORTH AMERICA, INC.