Hydraulic devices are always driven by pressurized fluids, either dynamically or statically. When either an external or internal force hits the pressure-bearing surface (often the piston surface) in a hydraulic device during operation, the fluids will always reactively form a resistance to change the motion status of the piston. The impact of such motion changes will always induce some significant pressure spikes due to the flow momentum and fluid compressibility. Those impact-induced pressure spikes will be propagated to the rest of a hydraulic system, resulting in instability in many operations, and therefore should be removed if possible. As illustrated in Figure 5.1, a pressure spike is formed as a piston is pushed by an external moving load. This pressure spike will be propagated to the rest of the hydraulic system, often in a wave, and will decrease as the piston is pushed away from its original position.