Although there are policy calls for educational research to discover "what works" and thereby inform decision making directly, the research literature argues instead for research to have a "conceptual" impact on practice. Empirical studies also suggest that, when teachers use research, their use is conceptual; research influences the content and the process of their thinking, changing attitudes and perceptions and making educational decision making more intelligent. This study investigates the ways in which educational research has achieved impact on practice from the perspective of the researchers. A sample of highly-rated impact case studies in the UK's research assessment exercise (REF2014) were subject to content analysis, using qualitative coding techniques. Analysis shows that most research is "invisible" to education practitioners because it is embedded in educational policies, technologies, and services. This "invisible use" is unlikely to realise the conceptual benefits claimed for research utilisation. If educational research is to make educational decision making more intelligent at its point of use, it will be necessary to re-think current notions of quality in research impact.