The key being that the end is surprising. When done well, you get good comedy: "I've had a perfectly wonderful evening, but this wasn't it." "I haven't slept for ten days because that would be too long." When done badly, you get a garden path sentence: "The girl told the story cried." "The raft floated down the river sank." "Violinist linked to JAL crash blossoms" (which is a real headline, the one for which crash blossoms were named). As for the word itself, like most rhetorical terms, it was adopted from Greek (literal meaning: "beyond expectation") -- the ancient Greeks loved analyzing how to speak.