This video tutorial explains how to reverse a video clip on iOS 8 using a third-party app since iMovie version 2.1.x and below for iOS doesn’t have this feature.
Download The Reverser Cam App: https://itunes.apple.com/us/app/reverser-cam-backward-video/id915039647?mt=8
Previous Tutorial – Making Subtitles: http://www.imoviehowto.com/imovie-for-ios-8-making-subtitles/