Temporarily lock an iPhone or iPad to require your passcode instead of FaceID or TouchID
Someone I know recently got his iPhone stolen by a few street thugs. They stopped him, one firmly held him while the others were emptying his pockets. When they found his iPhone, they simply took his mask off and unlocked it using FaceID.
This is one of the most important problems with FaceID, TouchID and other similar systems. They may be great for identification but are bad for authentication.
By clicking quickly 5 times on the Lock button of your iDevice, you can temporarily disable FaceID or TouchID and your passcode will be required to unlock it.