The ending of the Manifesto is what marketers would term a call-to-action: the part of an advertisement that asks the audience to take specific steps. Except this is no advertisement. This manifesto is an intentionally dangerous document calling for worldwide revolution.
"The Communists disdain to conceal their views and aims" (Section4.68), Karl starts off, like a baseball pitcher winding up. "They openly declare that their ends [goals] can be attained only by the forcible overthrow of all existing social conditions" (Section4.68). If you watered down that forceful overthrow phrase by replacing it with a phrase like "effective change," this might still sound a little edgy, but it would still be just ivory-tower talk.
So then Marx turns his amp all the way up, drowning out any doubt. "Let the ruling classes tremble at a Communistic revolution. The proletarians have nothing to lose but their chains. They have a world to win" (Section4.68). The rhetoric is in full force.
And then we get the all-caps: "WORKING MEN OF ALL COUNTRIES, UNITE!" (Section4.69). Well, that's hard to miss. It's Marx's specific call to action: he wants working men everywhere to join forces and overthrow the rich. If asked about the dictatorships that have arisen from communism, he might argue that the revolutions to date have failed because they were not large enough: they did not include working men from all countries, and did not bring down all of capitalism, or all of the bourgeoisie.