A key stimulus of learning efficacy for students in the classroom is active participation and engagement in the learning process. This study examines the nature of teacher-student and student-student discourse when leveraged by an interactive technology--Group Scribbles (GS) in a Primary 5 Science classroom in Singapore which supports rapid collaborative knowledge building (RCKB). We envisaged nine design principles for RCKB in the design of lessons and postulated a logic model that links and explains the effects of our design principles to the ultimate goal of learning efficacy. We presented a case study of a GS lesson which shows that GS affordances leveraged by good lesson and activity design could enable the students to have more epistemic agency. Students had opportunities to participate spontaneously in class discussions by fully expressing their ideas without inhibition. The technology effect is used to support instant formative feedback and interactions among students effectively.