When implementing digital technologies into mathematics teaching and learning situations, new representational and communication infrastructures arise that allow for new speech acts and actions. These new speech acts and actions are also referred to as 'representational expressivity'. Such expressivity arises in the intersection of representational and communication infrastructures. The paper presents an example of functions as covariation, taken from a task in a Danish 8th-grade classroom. Results show that infrastructures mediate students' mathematical communication. When students drag, continuous treatments between representations take place. The dragging not only influences the transformation and manipulation of mathematical objects – but also mediates the students' language. Loo king at verbs and adjectives, the students' words become more dynamic and focus on the movements of mathematical objects within the digital tool. From a competency point of view, the students must be aware of different mathematical registers in which communication takes place. When working with digital technologies, students must thus be able to co-exercise several mathematical competencies at once.