In this study, low-crystalline and nanostructured cobalt sulfide (referred to as L-CoS) were fabricated and chosen as model materials to establish precise structure-activity relationships between the local coordination environment and the observed catalytic activities in different media. By utilizing operando XAS, for the first time, we have established a full picture of the electrochemically driven restructuring in L-CoS toward both the HER and OER. For the first time, our operando monitoring reveals a stepwise local geometric rearrangement in L-CoS for the alkaline HER, wherein the in situ reconstructed metallic cobalt species with S-Co-Co-S moieties are responsible for initiating the HER. This differs fundamentally from operando monitoring of L-CoS for the acidic HER, where L-CoS remains redox reversible during the cathodic polarization, and the electrochemically driven optimization of low-valent S-Co-S moieties promotes the release of H2. As for the OER, the results corroborate that the formation of high-valent Co 4+ species is far more facilitated for L-Co-S compared to reference Co3O4 and Co-LDH. This is clear evidence for answering the open question: Why do the restructured oxide/hydroxide species derived from sulfidebased compounds always show better OER performance than their oxide-related counterparts? Notably, such related restructuring phenomena are also evident from ex situ characterizations of a series of reference sulfide samples under the same reaction conditions, i.e. highly crystalline CoS, CoS/CoS2, CoS2, and Co9S8/CoS2. Our work not only establishes a full picture of how to identify the true active species and sites of the investigated catalysts but also demonstrates that knowledge of precise structure-activity relations under operational conditions holds the key to efficient design.