which have a luff rope of some kind attached to the furler. On a top down, the furling line flies free of the sail completely and is not attached to it.

That said I've never used one. My sail maker talked me out of it - they hadn't much experience either but thought it might be tough on the spinnaker cloth since the thing is wound up by the end like wringing a wash cloth. There was enough work in my spinnaker that we didn't want to experiment. I used a sock - if you do go with a sock, make sure they make it long enough. ATN want's to leave them several feet short, so that the foot of the sail doesn't get caught in the funnel. But the result of that is that the sock will not snuff the lower few feet. I would prefer that it did, even if I have to go up and pull the foot out on the set.