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We've been playing with this for instant replay. But you'll need an extra person to hold the phone. But I like that it immediately allows you to replay the video, see in slow motion, or delete the video when you press stop (so you don't save every video from the bout).
It will take some coordination between the ref and the recorder so that the recorder perhaps starts recording on "ready" command, and doesn't delete the video until the the fencers have an opportunity to request for video replay.
https://play.google.com/store/apps/details?id=net.mnsquare.slowpro
It's free.