Four lines are all Emily Dickinson needs to overturn the ordinary arithmetic of living and dying. "A Death blow is a Life blow to Some" announces the paradox at once, and the rest of the poem works as a compressed, slightly teasing explanation. The "Some" in question are people who, "till they died, did not alive become." In other words, they were not truly alive while they were breathing. Real vitality starts only at the moment of death.
What kind of death is left open, which is part of the poem's interest. One reading is religious: death as the doorway to a fuller existence. Another is psychological. A "Death blow" could be any devastating shock, such as a loss or a collapse of the old self, that wakes someone who has been sleepwalking. The poem never settles the matter, and the vague, almost shrugging "Some" keeps those people anonymous. Nothing in the text says who they are, so the reader is invited to wonder whether the description might apply to them.
The wording in the middle lines is deliberately knotty. "Did not alive become" turns life into something you must become, not something you simply have, and the odd word order makes you slow down over it. Then line three tangles the logic: "Who had they lived, had died but when / They died." Roughly, it means that if these people had gone on living in the way they did, they would only have died at the moment they actually died. Their "life" was a kind of death all along, so there was nothing to lose. The word "died" appears three times in four lines, while "lived" and "alive" appear only once each. Death crowds the poem, and life arrives only in the final word group, "Vitality begun."
The sound of the poem supports this. The first line is almost all weight. "Death blow" and "Life blow" are set side by side, and the repeated "blow" lands like a hammer, so the two opposites are made to sound like the same event. "Blow" can also suggest breath, and this may be a quiet hint that something is being breathed into these people, though that is an inference and not something the poem states. The rhymes are uneven in a way that fits the subject. "Some" and "become" rhyme fully, but "when" and "begun" only half-rhyme, sharing a final consonant without quite matching the vowel. Dickinson is known for these near-rhymes, and here the slight mismatch leaves the ending a little unresolved. "Begun" is also not the form we would expect, since "began" is the usual past tense. The choice may have been made partly for the rhyme, but it gives the last word a faintly unfinished, ongoing feel, as if vitality has started and has not yet run its course. The lines also shorten from the long second line to a clipped, plain finish, and the dash after "become" gives a small pause before the argument turns.
The capital letters are typical of Dickinson. "Death," "Life," "Some" and "Vitality" are all capitalized, which makes them look like names or characters in a small allegory. The meter is loose, but it echoes the short, compact lines of hymns, a form Dickinson used often. That makes the poem's unsettling claim sound like a proverb or a riddle, and it comes with a clipped certainty.
The poem is significant because it packs a large idea into so little space and refuses to soften it. It takes the most feared event we know and says that for some people it is the beginning of living, which is a hard thing to read about people who are alive right now. The poem is not mainly about dying. It asks what counts as being alive, and it suggests that a long life can still be an unlived one. That question stays with you after the four lines are over, because the poem leaves it to the reader to decide whether they are one of the "Some."