When the speaker says that the "buzz saw snarled and rattled," they employ a literary device called onomatopoeia: a word whose sound duplicates the sound it describes. Both snarl and rattle are examples of onomatopoeia.
The speaker describes how the saw behaved:
As if to prove saws knew what supper meant,
[It] Leaped out at the boy's hand [...]
In this description, they use personification: the attribution of human qualities to something that is not human. In this case, the saw is given the ability to know something as well as to leap.
After the saw cuts through the boy's wrist, nearly severing his hand, the speaker uses metonymy—a substitution for something associated with a thing for that thing itself—when he says that the boy holds up the hand
as if to keep
The life from spilling.
It is not really life spilling out of this terrible wound but blood, but because blood is so closely associated with life, we understand what is being said here. The more blood the boy loses, the more likely he is to lose his very life. Blood is life.
The poem also employs a very matter-of-fact tone. Tone refers to the way the author seems to feel about the text's subject. After the boy cries pitifully to his sister, pleading with her not to let the doctor amputate his hand, the speaker simply says, "So. But the hand was gone already." Then, in the final lines, he says of the boy's family,
Were not the one dead, turned to their affairs.
In short, life must go on; everyone else still has responsibilities and lives to lead, and so they turn back to those lives almost immediately. The mood created here is almost serene, despite the awful events. There is simply nothing to be done about the boy's hand or the loss of his life, and so everyone else must go on with their own. It may seem unfeeling, but it is fact. It changes nothing to react dramatically.
The poem's title also uses an allusion—an indirect reference to another person, event, or text—through its quotation of a line from Shakespeare's Macbeth. When Macbeth learns that his wife has died, he launches into a monologue about the futility and meaninglessness and fragility of life. He says, in part, "Out, out, brief candle!" using a metaphor to compare life to a candle, something that does not last very long and which can be snuffed out so easily (5.5.26). We see this very idea play out in Frost's poem: a young boy's life is quickly and easily snuffed out, so to speak, as a result of an unpredictable accident. Further, Macbeth's response to the news of his wife's death is rather unemotional and matter-of-fact. He muses about life rather than grieves for her, and then he goes on to fight his enemy in battle. He must move on with his life, just as the boy's family must move on with theirs.