I first read about predicament bondage in the book "different loving". There was a lovely scene in there where the sub was tied, with her butt hanging off a chair. Clover clamps led from her clit to some weight hanging above. She could raise her ass up and take the pressure off, but by lifting herself up, she was pressing herself into her master's hand, pressing him deeper into her.

I've done something similar. my sub is tied on a table that's under our frame, she has to keep her feet elevated (think leg lifts), when she lowers then, it causes her tug on a rope that goes up and then back down to some clamps on her nipples, pulling on those. I use surgical tubing as a shock absorber so she can't really damage herself.

Another idea I toyed with was having her stand on her tiptoes, if her heels hit the ground, it would close a switch, causing an inflatable dildo to begin growing, she could probably put her heels down for a total of 60 seconds before the pump would have it big enough to cause her discomfort. I figured out the switch, and the pump arrangement, but never could find a decent inflatable to use. Still hoping though.