He uses the term 'iceberg' to visualize his approach in his story Death in the Afternoon. An iceberg floats about the surface of the ocean. A small portion is visible above the water. But its most significant mass remains beneath the surface. So then with good literary fiction: something vital must remain beneath the surface. The strength and power is beneath the surface, however explicit and powerful it may charge through the mind of the writer.
Of course, Hemingway did not invent the idea of omission, though he popularized it as a deliberate tool. Shakespeare omits the title couples' sexual history in Macbeth, and omits a great deal of the father-son relationship in Hamlet. He never tells us where Feste the clown is returning from in Twelfth Night, and why he ever left the side of the grieving Olivia. Shakespeare did not invent omission either. Homer's epics examine only core of ancient myth, and leave out much besides. The Bible, especially the Old Testament, leaves much to mystery.
Omission also appears in plays, like Terrence Rattigan's The Deep Blue Sea, and Kirk Lynn's new play Your Mother's Copy of the Kama Sutra. I won't express how Kirk Lynn's play uses it. Go to Playwrights Horizons and find out for yourself (closes May 11). In Rattigan's the Deep Blue Sea, the playwright never fully reveals source of sexual dissatisfaction between a separated couple. And he never reveals why an ex-medical doctor spent time in prison. In an odd way, the characters in Rattigan's play suffer from old-fashioned British restraint. But in Rattigan's hands it is a device that strengthens the story, rather than simply imitating mid-century British life.
Omission is a powerful dramatic tool.
I think, however, it's possible to go too far with the use of omission. It's not a fix-it device or a band-aid. Playwrights and directors and workshop-companies often describe a need for 'rough edges' and 'missing pieces.' These are necessary, but must be counterbalanced with the artists' responsibility to reveal us to ourselves--and to reveal themselves to us.
Perhaps more importantly, the iceberg metaphor deserves another look. An iceberg is made of frozen water. Spin it, tumble it over, let it melt a little. As it floats in the water, most of the iceberg will remain submerged beneath the ocean surface. Only a narrow fragment will peak above. From the perspective of the iceberg (or its viewer) it does not much matter which part is submerged and which part is above the surface. It will look about the same.
The difference between literary omission and an iceberg is this: it matters a great deal, when writing, what you leave below the surface, and what details you make explicit to your audience. Writing is not a frozen chunk of arctic. It is something wholly unto itself. The greater the writer, the better they know what to omit and what to state. It is an ultimate moment of craft.