Usually, the 'beat' I write to is the rhythm of the language (eg. my punctuation choices above), but the beat I'm trying hardest to listen to is the heartbeat of the story.
Screenwriters use the term 'beat' a lot, probably because movies unfold in real time. McKee has a very structured idea of which beats are important to the unfolding story. Probably because movies cost heaps per minute, so every unnecessary minute is clipped trimmed, folded into another scene or otherwise elided.
His idea: the heart of a story is forcing your protagonist into a tight place where she will be forced beyond her limits. The structure is one of pushing your hero, watching him react. The reaction will often make things worse, so that the next push will be harder. And again, harder still. Hopefully, at the climax all looks hopeless for our heroine, until she saves the day.
It's not a bad idea, and works as well for The Manchurian Candidate as it does for Bridget Jones.
Think High Noon:
Will Kane is happy, contented, the toast of the town. He and his new bride are looking forward to spending his retirement together. *yawn* But wait...
Word reaches Kane that Frank Miller and his boys are coming. Kane's conscience won't let him leave town. One by one, the townspeople prove to be unreliable cowards. There'll be no posse. No deputies. Kane, married that morning, is facing certain death.
Every act of cowardice that leaves Kane more isolated and alone is a major beat in the story. The minor beats are found in the back and forth that lead to those cowardly betrayals.
In my first draft, I'm watching my characters act and react, tracking those major beats to the story's end. In my second draft, I'll take these beats of conflict right down to the dialogue. I want *every* interaction to uphold the story in some way.
In a way, it goes back to Conflict vs. Complication. Every story has a central question at its heart: Will Frodo destroy the ring? Will Bridget finally choose Mister Right over Mister Wrong? Can newlywed and newly retired sheriff Will Kane stop the outlaws coming in on the noon train?
Resolving that question is our conflict. Everything else is complication with Two Important Caveats:
1) It may be necessary to set the conflict up. The first big chunk of Rocky is about what a crappy life the poor palooka has. It's necessary to understanding the stakes of his fight with Apollo. Just about all of High Noon is setup. We need to see just how alone Will Kane really is.
2) Subplots. These should have their own conflicts, beats and climaxes, spaced around the story to allow the writer to control the tension.
Aragorn's heritage and lovelife, Gandalf's death and rebirth, the dark spiral of Saruman's soul are all subplots that help mask the large amount of walking involved in Frodo's effort to nuke Sauron. Rocky's relationships with Talia Shire and Burgess Meredith (who I always expect to wave an umbrella and quack) keep us involved in that sorry man's life until his One Big Break comes his way.
To keep my own subplots from being pointless complications (or pointless, parallel stories) I try wherever possible to tie them back into the main plot. It can be thematic (for instance, all characters face a simliar decision, and in the subplots we get to see how different reactions play out). It can functional (there's a running conflict in Lee Child's ONE SHOT between Jack Reacher and his wildly inappropriate boat shoes. They're a constant hinderance, up until the final climax.). If they really are just complications, well, out they come...
In High Noon, two important subplots involve Amy (Kane's bride) and Helen Ramirez (his former lover). Both women love Kane, neither want him dead. They make different decisions about what sticking it out (thematic). Amy's decision is vital to Kane's survival (functional).
By the way, The Tiny Dynamo and I watched Dangerous Liaisons last night. Best damn education out there for beats of conflict in EVERY scene, for subplots that unite a theme and weave in and out of the central conflict. I'd forgotten how brilliant it was...