Poe wants everyone reading "The Pit and the Pendulum" to feel what the narrator feels. Considering the things he feels – fear, terror, horror, etc. – it's no wonder that our author has to resort to some drastic measures. Poe relies heavily on rhetorical tricks in order to bring his message home and strike terror into our hearts.
What do we mean by rhetorical? Well, in this case it refers to several writing devices, devices intended to keep us reading and build tension. Chief among these tools is repetition. It's there from the very first line: "I was sick – sick unto death with that long agony" (2). By doubly emphasizing "sick," Poe makes it very clear that our narrator isn't feeling very well – it also allows him to modify it, to give it more power. When he adds that phrase "unto death," well, we know exactly how bad he's feeling.
Repetition is just one of many tricks, but it is perhaps Poe's favorite. Later on in the same paragraph, he writes, "I saw that the decrees of what to me was Fate were still issuing from those lips. I saw them writhe with a deadly locution. I saw them fashion the syllables of my name" (2). Okay, so we know not only that the narrator saw these things, but we get an idea of how unrelenting and powerful each image is; each stands out, alone, in its own sentence; each has the power to exist independently of the others. Pretty neat.
Still, the most direct instance of repetition comes when our narrator is watching the pendulum come ever-closer to his body. Look at how Poe begins three consecutive paragraphs:
Down – steadily down it crept. […]
Down – certainly, relentlessly down. […]
"Down – still unceasingly – still inevitably down."
So not only does Poe repeat "down" at the beginning of each paragraph – he repeats it within each sentence, too. This kind of language is totally, unapologetically dramatic – some would say, to a fault. There's no question this stuff is over the top, but then again, so is the world Poe describes.
(Bonus tip: If you really want to impress your friends, you can use some big words to describe Poe's favorite rhetorical devices. Repetition of words at the beginning of a phrase or sentence, for instance, is called anaphora. Another one to keep in mind: alliteration. That's what you call it when multiple words begin with the same sound (and, usually, the same letter). One good example: "In their voracity, the vermin frequently fastened their sharp fangs in my fingers" (32). "Voracity" and "vermin" are an alliterative pair, and "frequently," "fastened," "fangs," and "fingers" are also part of an alliterative sequence. Now you're all set.)
A lot of people praise "The Pit and the Pendulum" for its realism. Most of Poe's stories aren't really rooted as much in reality, and most of his narrators are usually helped out in some supernatural way (as opposed to, say, by the arm of a French general).
So how does he achieve this realism? Well, the things that are most real to us are the things we can see, hear, smell, taste, and touch, right? So Poe goes ahead and writes a story filled with sensory words, phrases, and images. Even the pendulum alone requires us to engage in almost all our senses: he sees it, of course; he hears it "hissing"; he feels the "vibration" it makes; and he smells its "acrid breath." The only thing he doesn't do is taste the darn thing (but he was so close, he may as well have).
The sensory experience, realistic to its core, heightens the horror and makes this story stand out among the rest.