Health-risk behaviour like physical inactivity is more evident in deprived neighbourhoods than in nondeprived neighbourhoods, and in the former knowledge is lacking as to what causes effects in interventions on physical activity. A possible contribution to physical activity interventions is community engagement, which has been shown to be effective for changing health-risk behaviour, but more knowledge is needed on "the active ingredients" or mechanisms that make interventions work. The aim of this study was to give more insight into the possible mechanisms within an intervention on physical activity using community engagement. The study applied a theory-based evaluation approach using theory of change to uncover the underlying mechanisms of a community-based fitness centre in a deprived Danish neighbourhood. Data were gathered from documents about the intervention, semistructured interviews with three front-line workers on the intervention and ten residents participating in the centre as either volunteer instructors or members. The following mechanisms of the intervention to improve participation and health were anticipated by the front-line workers; the creation of meaningful communities through social interaction, the presence of relatable role models, residents taking responsibility and feeling co-ownership and the experience of being of value as an instructor. Interviews with members and volunteer instructors showed that the anticipated mechanisms did facilitate participation and improved health; however, with some individual variations and with the physical and mental benefits of the particular activities also functioning as mechanisms for participation and engagement. Furthermore, the study found potential unintended consequences related to engagement, such as difficulties in balancing the needs of others with own needs. Findings indicate that both the social aspect and the activities should be prioritised, as should a continued focus on the inclusion of different residents in the area. Furthermore, unintended consequences should be considered and prevented through support for volunteering residents.