With this rule, Masello suggests a way to figure out what happens next if your story comes to a grinding halt. His advice is to close your eyes and imagine the story unfolding in front of you as if it's on a movie screen. He says that by letting your mind run free, you can play out different scenarios and figure out where the story should go from here.
To me, this sounds like a good idea at first. However, when Masello mentioned envisioning one's story as a movie, a red flag went up in my mind. I remember one book on writing in which the author had been advised to envision his book as a movie. (At the moment, I can't recall whose writing book this was.) He felt this was a horrible idea, because a movie story is all on the surface, viewed from an outside point of view, and there's no way to get inside the character's heads. If you write your story by imaging the movie and just writing down what you see and hear, you're missing all the other senses and all the internal conflict that makes a novel the experience it is.
By the way, that's why almost all the TV and movie novelizations and tie-in stories you read have deep multiple internal points of view. (For an excellent example of this, check out the Serenity novelization by Keith R. A. DeCandido (kradical) -- or almost any other of his tie-in books. End plug.) Each scene starts deep within a main character's point of view and stays there as the scene unfolds. It's one of the strengths of printed fiction over movies and television, so it's important to take advantage of it when writing a tie-in.
However, I'm pretty sure Masello wasn't going that far with this rule. I've read many of his novels, which have plenty of internal characterization. The point of this rule is simply to present one more technique that might help a writer get unstuck.
And there are many ways to get a story moving again. One writer -- was it Raymond Chandler? -- suggested that if you don't know where the story is going, throw in a man with a gun. You may not know what he's doing there and what he wants, but his appearance definitely introduces conflict and action. Another writer suggested having uncle Fred show up from out of nowhere. Crazy uncle Fred! Dangerous uncle Fred! What bizarre scheme is up his sleeve this time? Suddenly all your characters have to react to Fred's presence, whether they want to or not.
And in the end, who knows? Maybe envisioning your story as a movie will lead to a movie deal. If so, I'll be more than happy to go see it with you on opening weekend. But you're buying the popcorn.