The 80/20 rule is also known as the Iceberg rule, or Hemmingway’s Iceberg rule. Hemming way not the first person to follow this rule, but he was the one to popularize it with a name. The idea is that when you see an iceberg floating on the ocean only twenty percent sticks out of the water, and the other eighty percent is what you crash into that sinks your boat. So in the context of a story, the writer only writes twenty percent of the story and the other eighty percent of the story is inferred. Often, the 80/20 rule is oversimplified in the phrase ‘show don’t tell.’
Essentially, the text of a story is like a trail of breadcrumbs that you leave for the reader. The reader follows the breadcrumbs you left and creates the rest of the story in his or her mind. A really good example of this is how you experience a movie. When a character is angry, they don’t say, “I am angry.” They go out and punch something or someone. I guess, the incredible Hulk is a notable exception. But when he says, “Hulk angry,” he does back it up by smashing up a tank and tearing down a building. You don’t just see him say, “Hulk angry,” and then see him just move on to some other part of the plot. Hulk is a big angry ass whooping machine, and you gotta show him go out and whoop some ass. But just enough to get the point across and not derail the story.
Say you have a character named, Bill. You don’t like Bill, so you say he is an ass hole. But you need Bill to go to the store and pick some groceries for the nice old lady down the block. And he has got to do it without taking too much time. You could say:
This is Bill, and he is an ass hole. Now, he is going to the store.
This one is fine, but Bill seems very one dimensional. Or you could say:
This is Bill. He just got back from a diner where he berated the waitress and talked the manager into comping his meal even though he ate the entire thing. Now, he is going to the store.
This one is better. We know who bill is, and we have a pretty good idea that he is an asshole without just going out and saying it. Also, it is relatively brief and lets you get on with the story. Or you could say:
This is Bill. He just got back from a diner where he berated the waitress and talked the manager into comping his meal even though he ate the entire thing. Yesterday, he kicked a puppy after luring it to him with a milk bone that he still has in his pocket. Tomorrow, he is going to hang out in the neonatal ward of the hospital giving papercuts to newborns and rubbing them with cut lemons. He intentionally gives wrong directions to lost motorists. He has had five arrests for driving under the influence in the last two months. And he really loves farting at the dinner table especially when he is berating a waitress. Now, he is going to the store.
This one is just too much. It shows exactly what kind of an asshole Bill is, But the story has been derailed. It is no longer about him going to the store. It is about him being an asshole.
So that is how the 80/20 rule works. It allows you to write a clear and interesting story without going crazy with the details.