It maps the ups and downs of the characters and the audience as they are spellbound in hoping and fear. This X axis also shows what's at stake, in other words the biggest hope (+) and the greatest fear (-). It's important for the audience to know what the stakes of a story are in order for them to engage with the character's journey.
So continuing with the Dragonweave of Back to the Future, I thought I'd map where these hopes and fear stakes are introduced in the story.
(For those who want to work on their own stories in this way, I'm using VUE, Visual Learning Environment, it's a mapping software put out by Tufts University. What I like about it is that allows you to sort your elements onto various layers with on/off visibility and it's free. The dragon is my dragon shape on the locked bottom layer.) Click to go to the VUE WEBSITE
The hopes and fears are also implied narrative questions. Will the hope succeed or will the characters suffer our worst fears? Each of these are two sides of the same coin with the audience always rooting on the side of hope. The villain roots for fear.
You can see how the filmmaker continually reminds the audience of what's at stake so the audience will keep hoping and fearing totally engaged in the story. This is so true that during one shot, while the DeLorean is supposedly traveling close to 88 MPH, the car ISN'T MOVING!
Below is a blank template of the dragon that you can import into VUE to map your own story weave.