This video explains how to reverse a video clip on iOS 8 using a third-party app since iMovie 2.1.x and below doesn’t have this feature.

Reverser Cam App: https://geo.itunes.apple.com/us/app/reverser-cam-backward-video/id915039647?mt=8&at=1000lbPr&ct=rvcam

Previous Video – Making Subtitles: https://www.youtube.com/watch?v=CCnbp5xJHqs