Pixel OFF uses Amoled's unique feature that it can turn off individual pixels completely. It turn off some individual pixels, reducing the power draw by the display
Con Reduces picture quality
Because it overlays a very thin black grid, it is effectively reducing the resolution of the display...however the pixels won't be spaced out properly (for example instead of having 8 pixels evenly spread out, now you could have 4 regular pixels followed by a black pixel, followed by 4 more pixels and another black pixel. This will distort the image.