Just from curiosity, and instead of an Unsharp Mask, one might also wonder, of course, what happens if you invert the method - instead using Gaussian Blur to create a sharp mask, you instead sharpen to produce a blur mask. Use Effects, Sharpen, Sharpen More, rather than Gaussian Blur. After adding to the original, it produces, not surprizingly, an image which increasingly loses its edges, and its contrast. The image on the right (or bottom), above, is the result after about 3 applications of such a filter. The unsharp mask tends to highlight bright areas and shadows, increasing contrast in that way, sharpening the edges. So reversing the effect would naturally tend in the other direction, toward a flatter image of less contrast. In addition, it slowly 'heals', or seals up, edges.
 
Other things could be tried. Same method, but if you use Effects, Geometric Effects, Punch, of 1, instead of Gaussian Blur, it produces a sharpening toward the center of the image, and only slightly sharpens around the edges. Might be useful. Pinch by 1 does pretty much the same thing. Both tend to blur more toward the center, so giving the same sort of sharpening effect as blurring the entire thing. Punch blurs out, as it were, and Pinch inward. Blurring a different way, using Geo Effects, Ripple, 1x1 centered at 50 horiz/vert, produces a slightly rougher sharpening, which might be useful. At 5 amp x 20 wavelength, it gives a sort of 'watery' sharpening. Or Geo Effects, Spiky Halo - offsets at 0, the rest all set to 10 - gives an even more interesting sort of 'watery' sharpening. Or try a Geo Effects, Pixelate of 4x4. Gives a sort of 'denim' texture to the sharpening. Blur the dark mask, Effects, Blur, Blur, for a dull brightening. If you use a Geo Effect, Horizontal Perspective, of 3, it produces a slight lighting and sharpening from the right, and not so much on the left, though it does introduce a possibly objectionable edge glow. And so on.
 
What if, instead, you didn't subtract the modified image, but did something else? Instead of subtracting, what if Average were used? Let's take Horizontal Perspective, but this time at a setting of, 7. Image #1 is the original, #2 is the perspective version, and combine with Average, say with a bias of 1 or 2. Then add another step. Create a second mask by subtracting this averaged result from the perspective version. Then add that dark, second mask, to the original (as before). Aside from having to crop the top and bottom, slightly, it gives a shaky blur, more intense on the right side.
 
What if, now, you were to invert the copy (Negative), blur, and then Average? You'd come up with a semi-embossed gray mask, emphasizing sharp borders. If you averaged with the original again, rather than adding, and applied a fade correction, it would look similar to Unsharp, except for 'graying out' slightly, bringing together slightly the brights and the darks. Now, what if instead of just keeping the gray mask, you inverted that, as well? Try this:

    

The image on the left is the original. The image on the right (or bottom) is the modified version just created (modified a second time, as well, not just once).
 
It gives a sort of 'soft focus', but where the edges tend not to be so blurred away as with the blur or soften filters. It also, because of the averaging, and the gray of the mask, tends to reduce contrast a bit, making the whites a little grayer, and the dark shades a touch lighter. To conclude from above, this is perhaps the more pleasing softening method, rather than simply Sharpening the mask from the manual method, previously. As a single method by itself, too, and forgetting the inversion of step 4, say if you have, for example, a photo taken on a bright day with lots of dark shadows, this might be useful upon repeated application in revealing more of the objects concealed in shadow and pushing down highlights, while simultaneously tending to sharpen edges with a very slightly embossed effect. So - a lot of possibilities, either with this variation, or more so with any assorted other variations you might discover from some combination of images using, Arithmetic.

It came to my attention when initially seeking help with this, that some might strongly object to the use of any inflection of the word, unsharp - such as unsharpen, unsharpening. They fear confusion awaits, as it already is a name which seems to suggest the opposite of what the filter produces. I've tried not to equate the two. But I don't agree, either, that it should be cause for concern if say, unsharpening, is mentioned, as a word. The method is not its result. It produces a result. And many methods seem counter-intuitive, at first, in all sorts of things in life. The method, at least in photography, uses a blurred image to help produce a sharper print. Once one understands how clever that might be, the rest is understandable.
 
I realize that there is some self-reference involved in talking about sharpening an image, by using something not sharp. It could be confusing if phrased - the sharper image, produced by the unsharp mask filter, is created by unsharpening the original in order to produce a sharper mask to be combined with what had been, by comparison, a previously less sharp image - in order to sharpen it, etc. And so on. If one wishes to be confusing, it can be done.