@KoiwiGal - I think a lot of the time when they use shark cages it's because they've used some kind of bait in order to get a bunch of sharks into the area.
In that case you really wouldn't want to rely on a device like this. I don't know how much it affects sharks but when they go into a frenzy I imagine all bets are off and they might just ignore the annoying sensation that's gently urging them to leave.
Even the cage would make me nervous to be honest. A full grown great white in a frenzy must be incredibly strong. You'd have to really trust the maker of that cage.