With Centra, the concern is that without visual contact students can easily become and stay stuck because the instructor can't see and diagnose their work. To make this class a success, it will be necessary to constantly cycle through the screens of the students, giving each in turn the "share application" power. This must be done unobtrusively (so that we're not wrestling with the medium and can concentrate on the subject matter) and quickly (so that everyone's work is seen yet the labs are completed soon enough to stay on schedule) and thoroughly, so that no one is neglected nor succeeds in "lurking."