The present work conceptualizes and applies the concept of "flow" as part of the didactic intention related to the quality of transitions between activities of a design experiment. The flow was used as the central protoprinciple in the developiment of a course on Special Relativity for high school based on a Design-Based Research (DBR) methodology. The work has involved the participation of researchers and public school teachers of the State of Sao Paulo, Brazil. Meetings, production and didactic interventions were collectively organized. The comparison between the desired flow and fluency observed by the students was analyzed as a result of the research. An auxiliary research on the understanding on the concept of event is also constructed and complements the research. Looking forward the generalization proposed by researchers in this area, the protoprinciple of the flow was presented as a potential idea for the development of didactic sequences. The flow's diagram was presented as a practical representation of the intentions of the developers and the flow instrument has shown as a method for producing data. These elements were tested in a cycle of research and are intended for use in other design experiments.