I don't think that's a feature in Poser 6 - I'm not near my installation so can't check, but I don't remember seeing that before. If you're doing still images, you can use the animation timeline as a multiple undo. Once you get to a point where you think you might want to keep what you have, step on to the next frame and continue from there. If you make a mistake, step back and retry. It isn't a step by step undo, but it's better than a single level!