The use of screencasts as an instructional technology is increasing rapidly in higher education. While there appears to be a consensus around students' satisfaction with the provision of technology-enhanced tools, there is limited evidence revolving around their impact in terms of knowledge acquisition. Moreover, the reasons why students choose to engage (or not) with these resources remain largely unreported. The study assessed the effect of using screencasts on undergraduate students' understanding and engagement with learning material in one of their modules. Customised screencasts were used as optional additional learning and teaching resources. Grades obtained in a test module (with screencasts) and a control module (without screencasts) were compared to gauge the impact of screencasts on knowledge acquisition. Furthermore, the reasons for students' engagement (or lack thereof) with the screencasts were explored using questionnaires. A modest but significant impact of screencasts on knowledge acquisition was found, and students' perception of the screencasts was overwhelmingly positive. Students suggested that screencasts should be kept short to summarise lectures or delve in-depth into complex concepts but should not replace whole lectures. Reasons for not using screencasts revolved around a lack of understanding of what the resources were and also a reported lack of fit between the nature of the tool and self-assessed learning style.