The film that Billy is watching in this scene is a film about war. Billy can see the destruction that war causes, and the way in which this is conducted "blindly" by people. The war appears to be a product of mechanization rather than a project with a clear objective. Whn the film is reversed, Billy is able to see the war going "backwards." Planes and tanks are protecting the city instead of destroying it.
This image allows Vonnegut to explore several messages. It shows that the war can have a different way of expressing itself if we are able to look at it from a different perspective. It also shows that the war can be manipulated by people, and thus, also stopped. Finally, it plays with the concept of time and continuity, which is a dominant theme in the text.