Holy cow, I wrote a book!
The hollow brush is a brush that doesn't do anything.
You can use it when you're forced to use a brush but you don't
As one example, you can use it as your class brush.
Then when your program stops responding and Windows decide
to do the "white flash" (see yesterday's entry),
it grabs the hollow brush and ends up not drawing anything.
(At least, that's how it worked on Windows 2000. Things may
be different on XP.)