Hyperbolic, Declamatory, Exhortative
Ol' Karl is full of rhetorical devices. After all, the whole point of this manifesto is to incite millions of workers to overthrow the rich, and it's a lot easier to do that if you can woo them with some fabulous turns of phrase. Here are just a few of the tricks up his communist sleeve.
Hyperbolic
"Hyperbolic" is a fancy adjective that just means exaggerating. Yeah, Marx talks pretty big. He often exaggerates his side of the argument, especially to make it sound like he's winning or knows exactly what he's talking about.
For example, he says, "Communism is already acknowledged by all European Powers to be itself a Power" (Beginning.3). Um, yeah—historically, that's totally a lie. But if you're reading a manifesto someone has just handed you in 1848 while you're sweating after leaving your miserable factory job, and you don't have an Internet to fact-check with, then you might just read that sentence and be like, "Hey, I'm gonna be on this team!"
To take another example of hyperbole, consider Marx's famous line "The history of all hitherto existing society is the history of class struggles" (Section1.1). Yo, Karl—all existing society up to then? It's not like the dude says, "Well, the history of most existing society is more or less the history of class struggles, to some extent, except there was that time in northern Russia where this isolated tribe of hunter-gatherers…."
Yeah, that kind of attention to detail might have strengthened Karl's argument, but with an uprising about to start in France and elsewhere, he didn't have time for that or an interest in it. If every sentence is qualified with a million maybes, then readers are going to get bored instead of reaching for their pitchforks.
Declamatory
To declaim something is to state it loudly, usually with an appeal to the emotions. Take this sentence: "The bourgeoisie, historically, has played a most revolutionary part" (Section1.13). Setting aside the brain-flip Karl's performing by calling the bourgeoisie revolutionary (his point is that they once replaced the feudal aristocracy), notice how he's just straight-up stating some fact about the bourgeoisie and giving it an emotional touch by saying most revolutionary.
Pretty much everything in the Communist Manifesto is declaimed rather than stated. It's not like Karl footnotes every sentence to a bibliography in the back of the book; he says it, gives his own reasons for it, and he wants to be the final authority on the subject. After all, who exactly would he cite as an expert on communism? He's the expert, right?
Exhortative
To exhort is to emphatically urge someone to do something. We can think of no better example than the last sentence, which shouts at you in all caps: "WORKING MEN OF ALL COUNTRIES, UNITE!" (Section4.12).
And before you get all, "Karl, that is rude. Don't you know that typing in all caps is the Internet equivalent of yelling?" Karl would just like to say, "DUH!!! I AM YELLING AT THE WHOLE WORLD!!!" Yeah, Marx wants to reach all workers, everywhere, and tell them all to join forces with each other. After all, the whole point of communism is world revolution, and we guess you've got to speak up if you want all those people to hear you.