Many L2 learners preparing for high-stakes, on-demand English language tests (e.g., IELTS, TOEFL) undertake classroom-based test preparation involving the provision of teacher written feedback commentary (WFC) on writing that simulates test tasks. The assumption is teachers' knowledge of both the language and testing system helps develop candidates' language/test-taking skills and familiarity with task expectations. Prior research has indicated features of WFC's content and delivery can impact on the extent and quality of student revisions, although preparation for writing assessment settings have yet to be explored. The present study investigated the effects of five WFC content and delivery characteristics (focus, length, explicitness, semantic function, and presence of mitigation) on three rehearsal essays written by eight candidates preparing for IELTS Writing Task 2. The qualities of content and delivery most associated with substantive, positive revisions included comments targeting Task Response, those 50 words or longer, when an explicit revision strategy was provided, the presence of mitigation through personal attribution, and question posing and criticism. The study found learners tended not to act upon descriptive end comments explicating written performance, praise, and comments below five words in length. The implications for teachers in classroom IELTS preparation contexts are discussed.