This paper focuses on codeswitching (CS) as a strategy employed by teachers in their EFL (English as a foreign language) classrooms in two commercial "bushibans" or "cram" schools in Taipei, Taiwan. More specifically, we are concerned with the perceived functions of CS in the EFL classroom context and the extent to which these constitute sources of conflict and tension. CS can be evaluated both positively and negatively. Policy makers, at both the national and school level, tend to view CS as undesirable within the language learning classroom, favouring an "English-only" rule. However, the prevalence of CS among experienced teachers seems to suggest that it has some positive function in the educational process. Functional and temporal analysis of transcribed classroom interactions reveals the strategic ways in which the teachers use CS to shape and guide their classes. We conclude that "English-only" is a lazy rule in that it means that we do not have to think about when and where CS is valid and useful and where and when it is pedagogically invalid and less than useful. (Contains 2 notes, 2 tables and 4 figures.)