When working in the Details Panel (Sharpening Section) to apply sharpness to an image I’ve created, to ensure it is only applied to certain parts of the image (the white areas above) I press the Alt (PC) or Option (Mac) whilst using the Masking Slider.
By using the Alt key I can see exactly where the sharpening will appear. White = Reveal, Black = Conceal.
If you don’t use the Masking Slider any adjustments made in the Detail Panel (Sharpening Section) will apply to the whole image.
BRASHLEY PHOTOGRAPHY 
Your comments are greatly appreciated