In this paper we present how the redesign of professional tasks in the teachers' formation of Secondary Mathematics Teachers influences changes in didactical analysis competency of future secondary school teachers. We draw on data collected from 3 groups of prospective teachers, using qualitative methods. We discuss how the training on the use of didactical tools to redesign tasks led prospective teachers to further develop their own professional competence to analyse mathematical tasks from a rigorous didactical point of view. [For the complete proceedings, see ED597799.]