With the proliferation of video-based professional development (PD) courses for mathematics teachers, understanding the role of the facilitators of these courses has become an emerging field of theoretical and empirical study. The purpose of this article is to address some of the challenges and opportunities that facilitation of discussions between mathematics teachers poses, by analysing catalytic episodes in which we were involved as facilitators. We offer a new construct, discussions sense, to capture the complex decision-making done by facilitators.