It saves your battery by two ways:
1. The best screens doesn''t need energy to paint black pixels
2. This live wallpaper actually does nothing. Nothing is nothing. Not "loading a black image". The back it''s blank, so your CPU will be thinking in other things, increasing your phone performance and decreasing the battery drain.
This is an Open Source project, so you can browse the source code and do cool things with it. You can also distribute the binaries of this project without worries. You''ll find the source code at http://code.google.com/p/black-npe-live-wallpaper