What does the buzz saw symbolize in “Out, Out—"?

The buzz saw in Robert Frost's "Out--Out--" symbolizes the mindless power of machinery that, when out of the control of man, can destroy human life.

Published in 1916 when Britain was already engaged in war, a situation that necessitated Robert Frost's return to the United States, this poem examines the duality of machinery; that is, the mixing of the danger and the productivity of machinery. 

And the saw snarled and rattled, snarled and rattled,
As it ran light, or had to bear a load. (ll.7-8)

In these early lines of the poem, it is the personified saw that performs the work, rather than the boy--much as with the young soldiers who fire weapons against their enemy. In the next line, there is a somber quality to the purpose of the saw, and the sense of finality that will come-- "And nothing happened: day was all but done."

But, just as in war, the machine can become such an extension of the man that it is as though the man is then the instrument rather than the machine. When this happens, tragedy can strike. In the poem, the saw

Leaped out at the boy’s hand, or seemed to leap—
He must have given the hand. (ll.16-17)

Underscoring the dangers of the machinery of war or of farm life, machinery that becomes the instrument of tragedy itself, the boy loses control of the chain-saw and his mishap costs him his hand, and eventually his life. Similarly, the machinery of war can cause fatalities that are accidental, and young lives are so quickly extinguished, as the title of this poem alludes,

Out, out, brief candle!
Life's but a walking shadow, (Macbeth, 5.5.23-24)

Clearly, the machinery of labor, much like the machinery of war, is capable of overpowering man and becoming a destructive force that places man merely in the shadows of life and death.