The advent of Face ID has ushered in a new era of convenience in biometric security. It’s also prompted privacy-minded folks to contemplate potential scenarios where police officers may attempt to unlock a confiscated iPhone X by holding the device up to the owner’s face.
Like with Touch ID, Face ID can be disabled quickly and discreetly at any time, even when the phone is in your pocket, with a quick and useful shortcut.
When disabled that way, Face ID stays off until you type in your passcode.

How to temporarily disable Face ID

To temporarily disable Face ID, do the following:
1) Press and hold the Side button and either Volume button.
2) After a second or so, up pops the “Slide to power off” screen with a pair of quick options for powering down the device or initiating a call to your local emergency services.
As soon as you summon this screen, iOS temporarily disables the Face ID feature.
Your face remains registered and Face ID is still turned on in Settings, but you can no longer unlock the phone with your face until your passcode is entered on the Lock screen.
3) Tap Cancel at the bottom or press the Side button again to dismiss this screen.
TIP: To prevent the “Slide to power off” screen from starting a countdown timer and playing a loud alert sound indicating an emergency call is about to be placed, disable the option labeled Auto-Call in Settings → Emergency SOS.
And there you go, boys and girls.
Temporarily disabling Face ID before nap time is the best way to make sure that no one can unlock your phone by scanning your face while you’re asleep—which is why you should never disable Face ID’s Attention Awareness capability.