Overscan emulates Reachability feature of iOS 9, which allows to move a running application window below to reach the top controls.
Overscan can be activated and deactivated at any time by simply clicking on the notification. With the settings you can adjust the "depth" of shift, as well as enable / disable the notification and automatic reset when the screen is turned off.
Attention! Not all applications are compatible with the Overscan. Some software (eg, the home screen) can become unusable after Overscan activation.