Downplay mode is a minor mode for Emacs. It allows you to apply a face to an entire buffer, excluding the region or the current line. I generally use it to focus attention on sections of a buffer during a demo or presentation.
By reading the docs for the mode directly.
The latest version is 0.1, and is available via
Marmalade,
MELPA, and
MELPA Stable:
M-x package-install RET downplay-mode RET
See here.