The rising prevalence of online courses and ubiquitous smartphone use pose challenges to researchers and instructors. Open questions concern the effectiveness of digital interventions under unsupervised non-lab conditions, as well as potential associations between interruptions, smartphone usage, and learning. We experimentally tested a web-based training intervention based on video examples and self-explanation prompts with 53 undergraduate teacher students (training condition, n = 27 versus control condition, n = 26). Despite the unsupervised non-lab conditions with potential distractions and interruptions, we found the expected effect on learning outcomes. More interestingly, this effect was completely mediated by self-explanation quality. Furthermore, the effect of self-explanation quality on declarative knowledge was moderated by the number of interruptions during the web-based learning. Moreover, we implemented a simple yet valid method to assess the learners' mean daily smartphone usage time. To do that, we relied on logging-functions most smartphones already have preinstalled. We detected moderate, negative correlations between the learners' mean daily smartphone usage and their task engagement (i.e., mental effort and lack of interruptions) during our intervention. Our findings emphasize how effective it is to self-explain video examples, and how important it is to not get interrupted during web-based learning.