I saw that too - and I think it is an answer to the question I pondered previously. If the user wants to maintain more tension on the jib without it coming inboard so much (i.e. higher wind), you hook that ball into the spring clip located on the travler. That adds one more purchase to the jib sheeting arrangement giving you more tension on the jib clew without increasing the load on the single sheet coming from the bridle foil (which would make the jib want to come to center).