Apologies but what do you need to do to preauth? Is this just the quick-double-pressing to open the Applet Pay wallet in advance? Do you still have to put your finger on it when making the actual payment?
Double press to bring up the wallet, then leave your finger on until the fingerprint changes to a phone with the message "Hold Near Reader to Pay". Then you don't need to leave your finger on when making the payment. Handy when you have to reach over a counter to "Tap" or need the payment to go through quickly (going through a payment barrier such as on the London underground). I assume FaceID will work the same.