Valeria and Emilia are two experienced high-school teachers, who position themselves as enthusiasts of using instructional mathematical videos in their teaching. They both see the videos as adjuvant to students' learning, but their respective lessons turn out to be very different, in particular with respect to students' participation. Teachers' knowledge and affective dispositions appear insufficient for explaining the difference between the lessons. Hence, we introduce the construct of transformational space to examine the decisions that a teacher undertakes to both make space for a new pedagogical resource when planning a lesson and fill this space when contingencies emerge during the lesson.