This is a nice pic on it's own, but I felt the connection to the challenge was a little too "loose" if I can say that. I think the background also threw me a bit too. Way too distracting. I want to be focused on the theme of your picture and on the challenge, not a mucky background. This would have been much better on a straight flat sheet of black leather, latex or perhaps silk. A number of thick ropes laid cleanly across the horizontal would have even been a better background.
Otherwise, I quite like the layout of the cuffs. The top one is a little suggestive and fits nicely with your theme.