Metal removal mechanism in electrical discharge machining (EDM) is revealed as shifting secondary discharges inside a cathodic root. Typical for EDM sinking process, the electrode couple of steel for cathode and copper for anode is used in this investigation. Micrographs of the discharge craters are taken from the surfaces eroded in the continual discharging processes with normal as well as reversed polarities. The apparent difference in crater morphologies between anode and cathode is investigated. The unique cathodic features indicate the existence of frequent spot expulsions of molten metal from the cathodic root during an entire primary discharge pulse. Shifting secondary discharges are discerned as the driving force to the special cathodic metal expulsions. Electrode energy equilibrium is analyzed to account for all the thermal contributors and the tendency of secondary discharges. The compliance of secondary discharges with long-disputed phenomena, such as the discrepancy between energy partition and metal removal, are demonstrated to be exempt from some of the conventional theories. Without the prior observation facilities, such as single pulse discharge, the method and results are made closer to a real EDM die sinking process. Such an insight into complex micro-erosion mechanisms is attempted to correlate better with the well-known consistent process behaviors.