I like this scene too, Cypher. She really gushes in that 4th clip, doesn't she?
I think you have to be relaxed to do it
If you look at these ones you posted in the whipping thread, you'll see that the girl thrashes around much of the time, but she goes very still when she relieves herself.
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http://www.cruxforums.com/xf/threads/whip-it-gifs-only.3326/post-451995
There is something about that release while bound, that relief in the face of torment, that I find interesting. That and the sense of humiliation of course.