The variable displacement type is particularly well suited in circuits using hydraulic motors where variable speeds and the ability to reverse is needed. The pressure of a pump is determined by it's manufactured capablilities. The ways you can harness the power of a hydraulic system is through the use of control valves . Both the tandem center and the closed center types are available in a three way or four way valve. BY opening or closing valves, you can control how much or in what direction a hydraulic piston moves.
Valves can also control many pistons working with each other at the same time.
This system keeps wear and tear on the pump down to a minimum. This takes pressure off the motor. This system is used in situations where the operating device needs to be moved by hand. This type of valve is used in a hydraulic system where the valve blocks the flow of oil from the pump into an accumulator. This valve takes the pressure off the pump and in neutral locks the cylinder in place with no load on the pump.