Existing research has highlighted the key role of observation and reflection in teacher learning during lesson study interventions, and how challenging it can be for pre-service teachers. This study focuses on the post-lesson discussion phase and on the observation of the research lesson of a lesson study process. It is carried out with early childhood education pre-service teachers who studied the Theory of Didactical Situations. The aim of our research is two-fold: finding evidence of the activation of pre-service teacher knowledge during the post-lesson discussion, and identifying mechanisms that may explain said activation as well as the potential development of their professional knowledge. To model pre-service teacher knowledge, the notion of teachers' praxeological equipment from the Anthropological Theory of the Didactic, specified in the Theory of Didactical Situations, is used. It is hypothesised that potential teacher learning is mainly connected to the identification of particularly relevant episodes during the observation phase, henceforth referred to as critical events. The analysis of a group discussion on three critical events shows the activation and development of mathematical and didactic components of pre-service teachers' praxeological equipment. However, it is found that it depends on the degree of dissonance these events can provoke, and on the quality and depth of the discussion that takes place around them. It is therefore concluded that the identification of critical events followed by productive discussion is a mechanism that explains teacher learning through lesson study, which can be enhanced by the intervention of teacher educators as mediators.