In two-stage exams students complete a test as individuals and then immediately complete it, or a similar test collaboratively in groups. Typically the majority of the students' marks (e.g. 85%) come from the individual component. The excitement of the exam seems to create very focused and useful discussions among the groups. It's difficult or impossible to find students who aren't participating (see if you can find any in the video).
Two-Stage Exams: Videos
Instructors: Active Learning Videos
Science Education Specialists
Full suite of videos available at https://www.youtube.com/user/geekgirl54/
University of Colorado Boulder Science Education Initiative