One of the first use cases I found for screencasting almost 10 years ago was to record short videos for my students to view (via a substitute teacher) when I was called for jury duty in New York City. I remember it very clearly. I recorded a video modeling the solutions to one of the more difficult problems from the homework in an Algebra class and then I had a short video that modeled a variation on factoring quadratics that was new to the students. From that point on they just worked in small groups on some problems and tried to puzzle through them. When I returned to class after two days (I didn't get selected for a trial) the students said they enjoyed the experience though I could not see myself making my entire class structured this way. It did lead to me having the students create explanations of problems which then of course fed to my interest in screencasting as a means for demonstrating understanding. Below is an example of how a teacher might record SHORT videos to support instruction when he/she is not able to be with students whether it is before, during, or after class.
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TEDxNYED: About Assessment