I love ball play, and it's always nice seeing balls in a humbler, trussed, or bound up especially if it gets the sub rock hard. But I find the sub is able to take more of a beating to it's balls when the balls are able to swing free and not bound up against a hard surface like this. As can be seen on this vid, the slapping must be restrained otherwise the pain can be too intense. The balls can better absorb harder blows when they are able to swing free.