Supplemental instruction, also known as Peer Assisted Study Sessions (SI-PASS), is a well-established form of peer learning that has been implemented in higher education institutions across the globe and that coincides with learning gains for participants. While the effects on learning gains have been extensively studied with quasi-experiments, the underlying mechanisms that make SI-PASS effective are less well understood. This study explored what benefits students thought SI-PASS offered and through which mechanisms. We studied this by interviewing 14 students who participated in SI-PASS during a field experiment that reliably found a significant impact of SI-PASS on performance. The students were asked to expand on if and why they thought SI-PASS was effective. Thematic analysis and independent coding indicated an interplay of three main drivers. SI-PASS was experienced as effective because it stimulated the use of effective study techniques and social learning. These drivers were facilitated and enhanced by a pedagogical climate that lowered the threshold to engage in collaborative learning and effective study techniques. These findings could help pinpoint what elements should be highlighted during the preparation of SI-leaders and what aspects should be monitored and tested when implementing or studying SI-PASS.