As pointed out by ‘tmiw’ on the MacRumors Forums:
Apparently if you have one of your Apple Pay cards open in Passbook and put your thumb on the Home button, the iPhone will start reading your thumbprint and then show “Hold Near Reader To Pay”. This seems to happen even when the phone isn’t anywhere near an NFC reader. I don’t remember this being in previous versions of iOS (though I could be wrong).
In any case this should save significant time at checkout as the Touch ID verification can be done while the clerk’s still ringing you up.
I haven’t used this yet, but I have tried it by opening Passbook. This could be good if the NFC reader is in an odd place, as it seems to hold this for about a minute before requiring a new fingerprint. Unfortunately, this may also encourage merchants to be lazy about reader placement and people wanting to take your phone.