I didn't watch the video (or read most of the page!) so it's possible I'm missing something, but the images show the wings having a propeller at each end, so I don't think a loss of cable tension would be catastrophic (at least not in the same way it would be if there was only one propeller per wing). The wing would presumably keep flying straight until the cable was tense again and it all started rotating again. Seems fairly self-correcting, with the second propeller.