If you press and hold both the lock button + the volume up button long enough to open the power-off screen, it will disable faceID (on iPhone) until you enter your passcode again.
Handy to know and easy enough to do discreetly in a pinch.
I know I've posted this a lot of times, so I'm sounding like a broken record, but you won't get a chance to do it. When the government came for me they pointed a loaded gun at my head. There was no way I could have moved my hands to start fannying around with my phone. I refused to cooperate when they threatened me, but they soon realized a new tactic and threatened my wife. You're probably tougher than you think. You might be able to take a spanner to your knee caps, but if the government puts a gun to your spouse or children (or even your dog), what are you going to do then? Certainly in the USA at least, I don't think the police are committing any misconduct by telling you they are going to take your wife outside and shoot her in the face if you don't give up your fifth amendment privilege.
On my phone pressing the power button five times quickly starts a 10 second timer and then it auto-calls emergency services and I'm pretty sure this is the default.
Handy to know and easy enough to do discreetly in a pinch.