Masello's tenth rule is about how to get started writing your daily quota of words, especially if you're in the middle of a project. Some writers stop in the middle of a passage that is going well the day before, figuring it will be easier to continue the next day. Others will take a look at yesterday's pages, fiddle with them a bit, and then use that as a springboard to help them begin today's work. And some writers never seem to have a problem; they're just able to pick up wherever they left off.
Masello's personal recommendation is to leave yourself a few notes at the end of the day, so you have an idea of where you'll pick things up tomorrow. He notes that the times he hasn't done so, he's often been convinced the next day that he had a brilliant idea that he should have written, and that he spends too long trying to retrieve that idea.
I've seen other writers talk about this as well. Lawrence Block, in particular, notes that when using the stopping cold method, he would sometimes face his typewriter the next day with an unfinished sentence such as "Her smile was like--" Block found himself straining to remember what simile he had in mind, and when he finally came up with one, it was never as good as the one he imagined he'd forgotten.
Now, while I find Masello's advice valuable, I have to admit that I've never found it all that necessary. I'm a nonlinear sort of writer. I don't sit down and write a story from beginning to end, or even a scene from beginning to end. What I do when I sit down for my daily quota is more like diving into the manuscript, shuttling back and forth, and producing new words all over the place until my page count has moved forward by my daily quota. So I've never found myself wondering how to get started. If the first scene isn't working, I'll just jump to a scene in the middle and continue playing around with that one where it left off. (As an aside, my works-in-progress are frightening things, filled with asterisks for missing lines and capital-letter notes to myself like "FIX THIS!" But only one other person in the world ever sees these drafts, and she's used to it.)
In the end, if you find that you have trouble when you get back to work the next day, you might consider taking Masello's advice here. I honestly don't see how it could hurt.