Leper describe the facts, but they way that he describes them adds a visual element; you are able to visualize exactly what happened. He describes Gene's motion that knocks Finny out of the tree as a two-part engine, a piston moving. With this, it is easy for use to visualize Gene bending his knees down, and the resulting wave that knocks Finny off. He also says that they were surrounded with "golden machine-gun fire" and that their profiles were "as black as death." This adds more imagery; you can see the dark profiles with the bright sunbeams striking out around their bodies. It gives a great image. But also note the words he uses; comparing Gene's action to "machine-gun fire", "black" and "death" adds a mood to it. It makes the entire event seem more ominous, threatening, and intentful, rather than the on-a-whim action that Gene insists it was. It is a poetic description that adds an air of morose morbidity to it, and this adds tension and suspense. So, rather than just saying, "Yes, I saw it happen. Gene knocked him out of the tree," Leper's imagery helps us to better visualize exactly what happened and how, and gives it an air of violence and intensity.