Windows 10 Mobile TP Build 10136 on the Lumia 1520

Windows 10 Mobile Technical Preview Build 10136 includes a new feature – Reachability.

In Windows Phone this means you can Press and hold the Start button and your screen will slide down so you can reach items at the top of your screen.

To get back to default position, users can tap the black space at the top of the screen, or press and hold the Start button again.

The feature is only available on devices larger than 5 inches, and seems to be Microsoft’s concession to those users complaining about reduced one handed usage on Windows 10 Mobile vs Windows Phone 8.1

Source