Question about 'mask brushes'

(I did look through the memories, but if it's there, I didn't see it- XD.)

I'm wondering how to use 'mask brushes' like these to make icons.

How do you get it so the 'background' image shows through like she did? I know it's *not* just a matter of 'stamping' the brush overtop.

Any handy tutorials, or anyone willing to explain it to my stoopid holiday-weekend brain?

Thanks for any help!