2nd and 11 on the 16 yard line:
In response to our tail motion, Duke rotates their secondary to bring the play side safety up into run support and then put the back side safety into a single-high safety look. You're right, the A-Back gets behind this run support safety, but it also shows how Duke has prepared for this play action look: the back-side safety immediately realizes that he is the guy who has to cover the A-Back or we are going to score and he runs over to cover him right away. I think JT is reading the safeties here to determine which side of the field to throw to and correctly identifying he should throw to Smelter on the Post. I think what happens is that as he's getting ready to throw into the endzone JT sees that the corner has really tight coverage and he chooses not to make the throw and scramble; Smelter doesn't get separation on his break. I think if the back-side safety didn't immediately turn and run towards the A-Back you would have seen JT turn his way.
It's an interesting contrast between the first two drives and the second half, JT is being extremely safe with the ball here.