Some poets want every line in their poems to pop. And in their better efforts, most lines do: the poem goes off like a string of firecrackers. But if a poem is to make the earth move under a reader’s feet, even a cherry bomb is insufficient. What’s needed is a more seismic sort of power. And the source of such power that's readiest to hand is a subject.
Consider this well-known poem by George Herbert:
The Collar
For most of its length, this poem is about what the speaker sees as the oppressive yoke of moral (mainly religious) stricture. Although it's replete with local strokes of imagery, language, and music, these are all delivered in the service of an extended rant that grows ever more “wild.” But then the poem's fast-freight momentum piles into its closing lines, where it’s converted to a momentous acquiescence in the will of God. These lines aren’t remarkable in themselves (their modesty is diametric to the tenor of another kind of ending, one represented by, say, “That dolphin-torn, that gong-tormented sea”), but their force is overwhelming.
The coup of “The Collar’s” ending is, like any artistic triumph, unrepeatable, but that doesn’t mean it’s not instructive. In pondering it, I find myself thinking of a golf swing. Even the strongest hands can’t drive a ball 300 yards. The power for that comes, rather, from the large muscles of the legs, back, and torso, which transmit it through the levers of the arms and the hinges of the wrists to a particular dimple on the back of the ball. Without the “large muscle” of a subject, a poem can only hit so hard.