Shibari (or kinbaku-bi)

It's a curiosity: why do some people like to be tied up? Why do some people like to do the tying? Clearly there are several aspects to it: beyond the subordinate/submissive versus the dominant thing (which is definitely roleplay for many if not most people) there is clearly a strong visual aspect. The way a coarse, tight rope pulls into smooth female skin is special - particularly if it's forming some geometric pattern against sensual curves of a body. There's also the directly sexual angle - a tight crotchrope pulled into soft genitalia or bound breasts is physically stimulating as well as being a visual turn-on.