Purpose: Mobile-assisted language learning (MALL) apps such as Duolingo have great potential for promoting learners' motivation to learn a second language (L2). However, little research has investigated how this motivational impact takes place. Additionally, despite the flexibility of mobile learning, most existing studies are conducted in classroom settings, with less attention paid to out-of-school technology usage. Design/Approach/Methods: To address these gaps, we present a model based on self-determination theory and propose the idea of "motivational transfer" to explain the psychological mechanism underpinning the impact of technology. To examine the model, we conducted a case study with 20 Year 8 Chinese junior school students who used Duolingo to learn English as a foreign language (EFL) after school for 6 weeks. Findings: Questionnaire and group interview data support our hypothesized mechanism--learners' activity-specific intrinsic motivation (IM) for using Duolingo and their underlying psychological need for autonomy and competence can be transferred to a more general level, thereby enhancing learners' global IM for L2. Originality/Value: The proposed theoretical model expands our understanding of how digital technology stimulates learners' L2 motivation; it can help L2 educators design better technological affordances to promote learners' motivation both in and outside the classroom.