If you think about it a little, you'll see that this concept can be used to create many useful tools quite easily.  The ones that come immediately to mind are a darkening tool, a sharpening tool, and a blurring tool.  Photoshop has these already (the Burn tool, the Sharpen tool, and the Blur tool) but they don't work as well as you'd like, and they are very hard to reverse or adjust.  That's because they aren't implemented as actions using layers.

So let's begin by creating a counterpart to the Fill Flash tool, which we'll call the Dark Flash tool.  Actually, we can use the the very same steps as in the Fill Flash action, except for the Curves Adjustment Layer step.  So what we need to do is (a) duplicate an existing action, and (b) change the parameters of a particular step in the action.  (If you didn't create the Fill Flash action last month, click here to see how it was done.  You'll need it.)

Begin by opening any image.  In the Actions palette, find and click on the Fill Flash action so that it is selected.  Click on the little round flyout menu button at top-right, and from the resulting menu choose Duplicate.

The copied action will appear just below the Fill Flash action, and will be named "Fill Flash copy."   Double-click to the right of the word "copy" to bring up the Action Options dialog.  Change the name of the action to "Dark Flash" and select Function Key F5 with Control checked.

At this point the Dark Flash action does exactly what the Fill Flash action does.  Now we have to change the Adjustment Layer step.

To do that, display the steps in the new Dark Flash action by clicking the little arrow next to its name.  Since all we have to do is change the parameters in the Adjustment Layer, double-click on that step.  A "New Layer" dialog will open -- again, change the name to "Dark Flash" and then click "OK."  Once you do, the Curves dialog will open.  Drag the marker on the curve two squares to the right, adjust the Input box value to 191 and the Output box value to 128, and click "OK."

Studying the Curves dialog, you'll see that the adjustment we selected is just the opposite of that for the Fill Flash last month.  There we raised the output for 64 up to 128; here we lowered the output for 191 down to 128.  The brighter areas are reduced much more than the darker areas.

We need to do one more thing to that layer, so select the "Make adjustment layer" step again and click the record button (2nd from the left at the bottom of the palette).

Now open the Layers palette; you'll see that the Curves Adjustment Layer is already selected.  (Don't worry that the name hasn't changed yet.)  The Blend Mode is currently set at Normal, which is the default.  Click the drop-down arrow to display the list, and select Luminosity (it's way at the bottom).  The result is shown on the left.

Now stop recording (the left-most button at the bottom of the Actions palette.)

So why do we want the Blend Mode to be Luminosity?  Well, if you darken the image substantially using Curves, it can cause a color-shift.  To prevent that we use Luminosity, which says in effect "for each pixel, use the Hue-Saturation-Brightness approach, and alter only the Brightness."  We could have done the same thing for the Fill Flash tool (and it wouldn't hurt to go back and do that), but the color-shift is less noticeable there.

Anyhow, we're done now, except for cleanup: close without saving the image you originally opened (because the image got modified).

So how does it work?  Just like its ancestor, the Fill Flash tool.  Consider the following

Image 1 is the original.  The rest of the image is properly exposed, but the elderly woman's makeup caused a slight overexposure of her face.  To make Image 2, the retoucher pressed Ctrl-F5 and then swished with the brush once over the face.  Next he pressed "x" to toggle the foreground color to black, pressed "0" to increase the brush strength to 100%, and made the brush smaller by pressing "[" a few times.  He then dabbed once on the small forehead area above the nose to restore the original.  In all, a few seconds work.

The image is not finished, of course.  The teeth need whitening, the left eye needs to be opened slightly, and face lines softened.  Speaking of teeth, you can see that the Dark Flash tool will darken teeth as well.  This could have been fixed just as the forehead was fixed, but since the teeth still need more work, the retoucher left that until later.

Let's do one more example.  The Dark Flash tool can be used for easy, hand-tailored vignetting as well:

On the left is the original (cropped, the usual blur with mask, a soft-light enhancement, and the eyes brightened with the Fill Flash tool).  To finish off, the retoucher pressed Ctrl-F5 and brushed casually around the outside.  (Maybe he shouldn't have been quite so casual -- but you get the idea.)

Remember that the adjustment layer is still open when you've finished painting.  You can leave it there if you want, and go back later to make more changes.  Or you can just merge down by pressing Ctrl-E.  (If you do go back, be sure to click the Layer Mask once before painting on the image).  And remember, after each stroke with the brush, you can always press Edit-Fade and adjust the amount of dark you brushed in!

Once you have this tool in your kit you'll be surprised at how often you use it for little adjustments.  And come back next month for another tool built on the same principle!