The construction of dynamic multimedia products requires the selection and integration of a range of semiotic resources. As an assessment task for preservice teachers, this construction process is complex but has significant potential for learning. To investigate how weaving together multiple representations in such tasks enables learners to develop conceptual understanding, the paper presents an indicative case study of a 2nd-year preservice primary (K-6) teacher who created a digital explanation on the topic of ‘transparency’ for stage 3 children (ages 11–12). We focus on data gathered during the 3-h construction process including artefacts such as images, online searches, websites accessed and paper records used for planning; the digital explanation as product; audio and video capture of the construction process and pre- and post-construction interviews. Using multimodal analysis, we examine these data to understand how meanings are negotiated as the maker moves iteratively among multiple representations and through semiotic choices within these representations to explain the science concept. The analyses illustrate the complexity of the construction process while providing insight into the creator’s decision-making and to her developing semiotic and conceptual understandings. These findings allow us to build on the concept of cumulative semiotic progression (Hoban & Nielsen, Research in Science Education, 35, 1101-1119, 2013) by explicating the role of iterative reasoning in the production of pedagogic multimedia.