The synchronous charge extraction (SCE) interface circuit is unique among the existing piezoelectric energy har- vesting (PEH) power conditioning circuits, for its output power is independent of the load. The previous studies about SCE have assumed lossless rectifier and ideal energy transfer through the inductor; the detailed energy flow picture in SCE was absent. This paper provides an impedance based analysis for the PEH system using SCE interface circuit. Through qualitative analysis on the energy cycle, the electrically induced dynamics of SCE is broken down into three components: the accompanied capacitance, dissipative resistance, and harvesting resis- tance, which correspond to an additional stiffness, a dissipative damper, and a regenerative damper, respectively, to the mechanical structure. Quantitative analysis on the harvested power is also carried out. Experiments on practical PEH system show good agreement with the theoretical results. The new insight provided in this study help better understand the dynamics and better evaluate the harvesting capability of the SCE circuit among those options of power conditioning towards practical PEH implementations.