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That's funny, I actually tried to do as many pushups in a row as I can the day after hearing it. And I did the most I've ever done before btw :)
Why do you think it's specifically good for physical exertion? What about mental or emotional exertion?
I suspect; pain is a physical feeling, weakness is similarly an understanding of physical problems. By pipining the pain=negative weakness, you convince yourself that (temporarily) more pain is helpful to the cause of doing more pushups, rather than previously "not helpful" as you would have been treating pain and deciding to stop.
I have heard it work before for pushups or similar exercise by danielfilan
(side note: don't push too hard, trust your body's limits - it tends to know when its going to break)