The actual working process of a hydraulic impact mechanism involves the automatic motion control of three primary moving components: the piston, valve, and accumulator. As a result, the motion state within a single impact cycle becomes considerably complex, comprising 12 distinct states from the beginning of one return stroke to the next. Using the YYG250 hydraulic impact mechanism as an example, this chapter provides an in-depth analysis of the force conditions and variations in pressure and flow rate across these 12 states. The fundamental motion equations are derived, and the calculation methods for local resistance losses are presented, laying the groundwork for the development of a nonlinear simulation model in subsequent chapters.