Screen Curtain Toggle.
||Three Finger Triple Tap.
||Toggles the display on and off. Turning off the display is good for privacy. If an iOS Device is lost while the screen curtain is on, placing the device in "Lost Mode" does not turn it off. People who find the device will not be able to read any message that might be on the screen.
VoiceOver Basics: Lesson 1, Section 8.2