Perhaps the motion sensor and telling him to keep absolutely still would work well.  That's not so easy when you're being edged and you have to keep your knees bent at the same time. Hanging it from his balls sounds a real treat! High sensitivity, no point in wasting time here. I've been on the receiving end of one of those, trouble was Sir kept flogging not remembering he'd left it set on high. I was trying to indicate his mistake, not always sensible..., through the gag but got a raw back and a frazzled cock. "Oh, sorry" he said at the end laughing.....