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Reversing a series of frames is a useful effect. A basketball bouncing up and down, a flag waving side to side, a boomerang advancing and receding: These things are all examples of reversing a single series of frames.
Instead of creating the two complete series of frames by hand—one showing a ball falling, for example, and another showing the same ball bouncing back up—you can copy the frame series, paste it, and use Modify→Timeline→Reverse Frames to reverse the pasted frames.