The brightness control allows you to change the brightness of the reading screen by swiping up and down on the screen with your finger. Swiping down decreases the brightness and swiping up increases it.
You can modify the brightness control behavior in settings: turning it off, or setting it to “System” or “Overlay”.
The System brightness setting uses an iOS API to change the screen’s brightness. This overrides the device’s brightness setting for the duration of your current lock session; and will stay in force, even when you exit the app. To restore the brightness level to it’s normal level, lock and unlock the device.
If you don’t like how the System brightness setting works, you can choose the Overlay brightness option. This works the way the Stanza brightness setting did, by overlaying a semi-transparent black layer on top of the reading screen. Decreasing the brightness makes the overlay less transparent and increasing the brightness makes it more transparent. The advantages of the Overlay option are that it only effects the reading screen and you can make the reading screen darker than is possible with the screen brightness setting. The disadvantage is that it doesn’t save battery life as the System brightness control will.