How does this drone climb while upside down?

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A quiet weekend's RC plane and drone flying but we were visited by 6 Cessnas and a Piper from the Auckland Aero Club so things were quite busy for a while. And can you work out a drone managed to do an inverted climb in the intro to this video? I didn't run the video backwards and the sound track is unedited. (answer is at the end of the vid).
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