In a TED talk, Pixar writer/director Andrew Stanton said it should be like a pebble in a slingshot as the elastic is pulled back...
We want to see what will happen--what is the stone's target? Are we watching a Davd fighting a Goliath? Obviously our reaction will be very different if, instead of a giant, the target is a little boy.
In either case we want to know the basis of the conflict. And we want to know how it will turn out.
In other words, you have our attention and you have started to pull us into the story--we want to know more.
Although there can be variations in the pacing, you have to keep the tension going. Maybe the first shot misses. If the target is a giant, we will tend to identify with the person with the slingshot and we will worry about his fate.
If the shot doesnt miss, is it enough to kill the giant or might it just enrage him more?
I think it's a very useful metaphor, one that can help you build in tesion right from the start, and can guide you in tightening the story when you do the next draft.
(Want guidance on writing your book, from the first idea right through publication? Or your screenply, from seed of an idea through to something you'd be hapy to submit to an agent or producer or studio? You can get it in my book, "Your Writing Coach," published by Nicholas Brealey and available from Amazon or your other favorite bookseller.)
Will it hit the target? What is the conflict that