Tying to get this to work
"You can, however, add a Levels or Curves adjustment to a mask. Add a Curves layer, for instance, and drag it over the Mask thumbnail in the Layers panel so that you get the vertical blue bar. In the Curves dialog box, choose "Alpha" as the channel to be manipulated (rather than the default choice of "Master"). You can directly adjust the mask in this way, and it is effectively a non-destructive layer."
However when I create a mask layer and attach either a curves or level adjustment I see not visible effect to the mask, I would expect to alter the attached curves / level and then view the mask layer by alt-Clicking on it's thumbnail to see the effect of the level/curves. Am I missing something ?