Huawei P20 User Manual page 26

Gestures and Operations
Use knuckle to start recording: Knock two of your knuckles to the screen twice in quick succession
to start recording. Select Microphone, System, or None from Sound source based on your needs.
To end the recording, gently and rapidly double-tap the screen with two of your knuckles again.
You can view your screen recordings in Gallery.
Take a Screenshot of Your Entire Screen
Use combo key to take screenshot: Simultaneously press and hold the Power button and the
Volume down button to take a screenshot of the full screen.
Use shortcut switch to take screenshot: Swipe down from the status bar and open the shortcuts
panel, then touch to take a screenshot of the full screen.
Use knuckle to take screenshot: Use a knuckle to gently double-tap the screen in quick succession
to take a screenshot of the full screen.
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