You can only roll the die once. Alice and Bob have a dozen dice. They agree that if they roll a 4, that's the signal for "attack at dawn" and Alice has decided

You can only roll the die once. Alice and Bob have a dozen dice. They agree that if they roll a 4, that's the signal for "attack at dawn" and Alice has decided that she wants Bob to attack at dawn. Alice rolls the 12 dice, and at the agreed upon time, Bob also rolls his 12 dice. The sequence they get is 1,5,3,2,2,4,6,5,1,2,1,6. The 6th die is a 4, which is the signal that Alice wants to send. How does Alice signal to Bob that she wants him to look at the 6th die? If Alice can send a signal to say, "look at the 6th die", why wouldn't she just send a signal that says, "attack at dawn"?