Nearly 100% of hydraulic power steering systems utilize an "open center" control valve concept. Power steering fluid flows out of the pump down through some type of valve. When driving straight ahead, your system is typically flowing 2.5 gallons per minute through the system and back to the pump reservoir with something like 50 psi of system backpressure.
When you start to turn the steering wheel, you partially restrict that flow, build pressure, and the valve directs the pressure to the side of the assist piston that helps you steer in the proper direction.
The 1963 through 1982 Corvette and the pre-1965 Chevrolet B-cars had linear valves but they still were open center just like all the later rotary valve power gears.