It's not a nude filter, the artist did three versions, clothed, without panties and nude. Unfortunatelly I can't link to the source since it is "pining to the fjords" of e-hentai; don't ask, what that is supposed to be, but I think the artist's name was E-NERG or something.
There's something specifically enticing about their inability to bend their legs. Probably that any time they would want to, they can't. Tangential to bondage?
