Clipped, Cutting, Concise
Hemingway's writing leaves quite a bit up to the mind of the reader. And what Hemingway actually says isn't always what you'd think he'd say if he just wanted to be straightforward. Exhibit A, a typical scene sketch:
They were in the cave and the men were standing before the fire Maria was fanning. Pilar had coffee ready in a pot. She had not gone back to bed at all since she had roused Robert Jordan and now she was sitting on a stool in the smoky cave sewing the rip in one of Jordan's packs. The other pack was already sewed. The firelight lit up her face. (38.1)
Hemingway's set the scene with just enough of the bare essentials to form a mental picture, but he doesn't fill in who "they are" (you just assume it's the whole band), he doesn't describe in detail what anybody besides Pilar is doing, but his description of Pilar quickly focuses the scene, and the firelight being on her face is enough to evoke the illumination of the whole cave.
Also, the sentences feel abrupt – Hemingway could have said "Pilar, whose face was lit up by the fire, had coffee ready," or "Pilar, who had not gone to bed since rousing Robert Jordan, was now sitting…" and so forth. Instead, each detail gets its own sentence. In action scenes, the combination of Hemingway's choices of omission and his almost artificially short sentences can be a bit jarring:
Aiming at the center of his chest, a little lower than the device, Robert Jordan fired.The pistol roared in the snowy woods.
The horse plunged as though he had been spurred and the young man, still tugging at the scabbard, slid over toward the ground […] (21.5-7)
Here the chopped-up sentences break up a rapid and violent action, and we never actually get the information that the bullet hit the guy (though it did – he's done dead). We get the before and after, and the center of the action is given to the sound of the pistol (which would seem a less-important detail). The chilly matter-of-factness here is also a bit unnerving in relation to the nature of the action: a man died, and it's described with all the emotion of having broken a non-descript tea cup. Hemingway's style is often ascribed to both his journalistic background and his affinities for Modernism (capital M), of which it's one variety.
One last thing. When Hemingway chooses to break from this style at a few choice "orgasmic" moments (and yes, they're during sex), the contrast is all the more striking. Hemingway does this different ways, but the most usual manner is to repeat himself a massive number of times in epically long run-on sentences. Feast your eyes on this messy morsel of repetition and obscurity: "For him it was a dark passage which led to nowhere, then to nowhere, then again to nowhere, once again to nowhere, always and forever to nowhere, heavy on the elbows in the earth to nowhere, dark, never any end to nowhere."
You get the picture. Besides that nice little riff on "nowhere," there are also riffs on "now" and "muck."