The mechanism of a chemical reaction can be characterized in terms of chemical events that take place during the reaction. These events are bond weakening/breaking and/or bond strengthening/forming. The reaction electronic flux (REF), a concept that identifies and rationalizes the electronic activity taking place along the reaction coordinate, has emerged recently as a powerful tool for characterizing the mechanism of chemical reactions. A quantitative theory introducing new descriptors for characterizing reaction mechanisms is presented in detail and three illustrative examples are revisited. In nucleophilic substitution reactions the REF indicates that bond breaking or forming events may be leading the electronic activity whereas in the methanol decomposition reaction by copper oxide, the REF allows to discover that consecutive electronic reductions of copper together with bond breaking processes control the course of the reaction.