Saves battery by applying a filter that turns off pixels of AMOLED screens. AMOLED screens doesn't have a backlight at all. Instead, each little sub-pixel is like a tiny red, blue, or green colored light. If the screen is instructed to display black, it doesn't need to block any light, it simply doesn't light up any of the little colored sub-pixels. Black pixels save you a lot of power because those pixels can be turned completely off. There is a relationship between brightness and power, but to really save juice you need a true black.