This article explores external facilitation for school improvement; more specifically, it examines the practical work of external facilitators. It is based on a Swedish case in which facilitators at the Swedish National Agency for Education support low-performing schools. The article aims to develop knowledge about how external facilitators in their practical work both promote and prevent developed understanding and improvement capacity. Based on observations and documents, facilitators’ endeavours and critical situations in their work to support school improvement are identified. The findings show five different endeavours to promote development of understanding and improvement capacity. However, they also show how taking ownership, simplification, and model focus, three of five identified recurring critical situations, prevent such development. The conclusion is that there are several aspects for external facilitators to balance to promote school improvement and turn around low-performing schools. When planning for external facilitation, it is important to consider how the facilitators’ practical work can be adapted to the local contexts of those who are facilitated in order to promote their ownership of the processes. Without balance and adaptation, external facilitation can lead to deprofessionalisation instead of development of improvement capacity. © The Author(s) 2024