If you are in a situation where you want to instantly and securely lock your iPhone there are two quick and easy methods. Both of them instantly disable Face ID and Touch ID such that the phone can only be opened manually by entering a passcode.
The first can be done stealthily but requires you to be in physical contact with your device. The second method can be done from across the room using just your voice (in the event the phone is out of your possession).
Method 1 (physical):
On newer phones (iPhones 8 and later): squeeze the right side power button and either volume button simultaneously.
On older phones (iPhone 5s to 7): click the power button five times rapidly.
Method 2 (voice):
Simply say “Hey Siri – Who’s phone is this?”
The phone will show the owner’s contact card but it’s otherwise frozen until the correct passcode is entered.
The are a myriad of scenarios where this feature could come in handy. It’s worth your time to practice each method until they are memorized and reflexive. Hopefully you will never need this information but it could be crucial to securing your personal information.