Marina Sawdon, a lecturer in Medical Education at Durham University asks voting system questions as part of the lecture each week. Some of those questions address topics covered in previous weeks, not just the topic covered that day. She is able to use this to demonstrate to students that they are retaining knowledge. In fact, the number of correct answers goes up when she re0tests students on earlier learning. Marina badges this as an additional form of feedback to students and she has had very positve reactions from her students to these interventions.
See http://onlinelibrary.wiley.com/doi/10.1111/j.1365-2923.2009.03332.x/full for a full article on her work in this area.