Recent discourse about the impact of university research has focused on "what" research impact is and less on "how" it manifests. To explore these two questions, we draw on knowledge theory and build on the distinction between explicit knowledge and tacit knowledge. We propose that tacit knowledge and knowing are necessary conditions for impact to occur and illustrate our conceptual model through two highly ranked research impact case studies as well as an analysis of 26 research impact cases from the UK and Australia. Our findings and model extend our understanding of research impact by helping researchers, university leaders and research funders to increase the impacts of research by focusing more on how such impacts are achieved.