When you storyboard at the onset of building a communication, you craft the outline of the story you intend to tell. As the name implies, reverse storyboarding does the opposite. You take the final communication, flip through it, and write down the main point from each page (it's a nice way to test your horizontal logic as well). The resulting list should look like the storyboard or outline for the story you want to tell. If it doesn't, this can help you understand structurally where you might want to add, remove, or move pieces to create the overall flow and structure for the story that you're interested in.
Stay tuned for the final post in this series, which will be on the value of soliciting a fresh perspective.