I just know that when I place my phone next to the payment sensor, the phone switches to "pay mode". So if my thumb is there, the action is just put phone next to sensor, put thumb on fingerprint, in one fluid motion. For my wife there's this, place phone next to sensor, lift phone up to look at it, then put it back down. Often times lifting it up causes the payment system to timeout.

Perhaps there's a smoother motion, double clicking, then face-id then press. To me, the finger print sensor just means I'm tapping my phone on the sensor.