Actually, it's pretty easy. YOu have your start frame and your finish frame, with everything else carried out behind the proverbial scenes. Let's say your start frame is 1 and your finish is 20: twenty frames of animation. Highlight Frame 1 so you're capturing all the contents, then shift-drag it over to frame 40. Between 20 and 40, you should now have a reverse. If you want it saved as a separate sequence, just delete 1-19.

docandraider.com -- the collected cartoons of Doc and Raider