"Lesson explaining" has been developed in China from an evaluative resource to an effective form of teacher professional development with the value of emphasizing teacher reflective practice. This paper begins with a general description of lesson explaining. Then an example of "explaining" a mathematics lesson for teaching probability is described. The analysis shows how the individual teacher develops his own subject matter knowledge of probability and how the professional community develops their pedagogical content knowledge during the professional activities based on the task of lesson explaining. In the concluding remarks, lesson explaining is described as a model of professional development for in-service mathematics teacher and is used as the basis for an alternative explanation for the Confucian heritage culture (CHC) learner paradox and the CHC teacher paradox.