I'm using TOR so my ID changes

This one is nasty: the trap consists of an arm-length tube with a grinder/sander wheel at the back spinning at high speed, and the wheel axis can move forward and backward in the tube. The goal is to make the axis reach the mouth of the tube. The way to make it move is to resist the spinning of the wheel, and the only way to do that is to press on it with your own flesh. While the grinder encounters resistance (while your flesh is pressed against it), it very slowly moves forward in the tube. Otherwise, it slowly moves backward at the same pace. To lose the minimum amount of flesh, you need the willpower to keep your flesh constantly pressed against the wheel without rests. Once the wheel reaches the mouth of the tube, the door to your cell unlocks.