Question about using the camera with voice over in IOS 10

Hi all. Something's always puzzled me. When I've seen both Apple ads, and other visually impaired or blind people take a photo,I hear "focus locked", before the sound of the pic being snapped. I for the life of me can't get the camera to do this behavior in any situation. I will use selfi X which I do enjoy, but yesterday I tried using the stock camera. I hard one large face, centered. And that was it. No focus lock or anything, so has anyone gotten this to work? I'd like to do that, get the behavior to happen if it's supposed to I just don't know what i'md oing wrong. thanks for any help. btw, this is an iPhone six s plus IOS 10.2.1 I think, and I am sure this will happen on any of the other models, though if you seven owners have something new, tell me.