It looks to me that the two pieces are to be used in tandem, one on either side of the spring. But used for what is beyond me. I think it would be equally useless for knitting and crocheting, just like it appears to be useless for tatting. Is there some sort of bobbin lace that could benefit from it? I know it's nothing like a bobbin, but the tension aspect makes me wonder. Although I've never knotted a fish net, I've watched enough fishermen do it, and this contraption would also be quite useless for that. There's no place for it in macrame or Chinese knotting either. Sheesh, now it's driving me nuts. Antiquarian, go smack yourself. I sure hope we get this figured out.