She relates it to the way a symphony is constructed and that seems good analogy to me. Certain expectations have been set up and then resolved. There are a dozen subconscious cues in the structure of the piece to say that we are reaching a conclusion, not because the composer ran out of tunes at that point, but because of the way the piece is 'built', though we don't really notice it. The coda gives a foreshadowing of what comes next, and reflects on what has been. I'm not sure the book segments out as neatly as she implies, but I'll present her analysis of the first 'movement'.
Magic enters the safe homely world of the party, a quest is begun.
Frodo's only aim at first is to get to Rivendell. After that, greater ones than he will decide what is to become of the Ring. Jones describes them as the dead hand of the past reaching to disturb the present. They're carrying the left-over hatred of the wars of Angmar. This is a foreshadowing of the way the Ring has reached out of distant history, loaded with the weight of an old evil which will flourish in the present.