I expect that there might be a great deal of difference between an externally imposed threat and a self-imposed threat. "Start working, or I'll do something nasty to you" never seemed to motivate me very much if the task was at all difficult. Threats tend to make me less inclined to attempt whatever task it is that I am being threatened to do, rather than more.

And it seems stupid to precommit to a penalty for not achieving a goal that I think I'm likely to fail at.