[Rosenberg]

I was just going to suggest the "fingers" theory of Finwe, except I always thought of this knowledge of Sauron's will as more of an intuitive devise that allowed a limited degree of creative free will. Sort of like telling your employee to do so-and-so, but not specifying how you want it done. Not that I'm saying Sauron couldn't give very specific instructions if he needed to. The exact moment to attack, for instance. Hence the signal.