Looks like a nicer version of a lungeing attachment
, in which case it would snap on to each ring of the bit under the horse's chin. You would then attach the lunge line to the ring in the middle, so you get equal pressure on both sides of the bit.
I've never used one, but I don't think it would be all that better than running one side of the lunge line through the bit ring, over the poll, and hooking the snap on the other side (except that you don't need to change when you change directions). I prefer to not attach the lunge line to a bit at all when I lunge.