The photo above is applied a Masking with quite big value (77). But what it means for the processing exactly? Well, when you press the Alt key and move the slider, the image turns into black and white, showing the mask. Black areas are the areas unaffected by the sharpening tool, while white shows the areas where the sharpening is applied:
Moving the slider to the left (decreasing the masking) adds the sharpening to more more elements of the image:
You can use Alt key with all sharpening sliders (Amount, Radius and Detail). To me using it with the Amount slider seems to be quite useful. Look at the detailed view with a specified amount of sharpening:
The same fragment with Alt pressed:
Using the black and white view instead of colour helps to reveal the details of the texture better and apply the right amount of sharpening.
I hope someone will find it useful.