Couldn't you do the same thing with threshold Pick the blackest pixel Open curves move the endpoint to the pixel selected say 15 for the sake of the arguement and move it another 10 to the right thus steepening the curve? I have found that some people will recommend a convoluted method for the sake of being "original"
I think Jeff's solution is more direct. You can also alter the values in the foreground color picker if so desired (he mentions the default selected color is used here), plus the fuzziness would appear to make this a much smoother selection and one which provides a specific value to target. The Threshold trick seems to take more steps and provide less options. I also like Jeff's technique as it puts the alterations on a layer and uses a simple blending mode to achieve the effect, providing even more control (if necessary) with layer opacity. So, Jeff's technique seems far less convoluted and more controllable.