The commas and periods throughout "Out, Out - " are indications of slight pauses or the end of a sentence, as in most written literature. The colon in the ninth line ("nothing happened: day was all but done") separates the two parts of the thought, but they are related to each other.
The critical use of punctuation comes in the latter half of the poem. The device is called a "hard caesuras," it indicates a sudden break in the action being recounted, and Frost uses a dash to indicate where such emphatic interruptions should be inserted. When the poem is read aloud, which is the intended method of presentation for all poetry, these are the points where the reader's voice should reflect the momentous changes in the situation.
The first appearance of the technique is "the saw...seemed to leap - He must have given the hand." The narrator has expressed concern about the boy using the man-sized equipment, but has reported that the boy successfully continued to do so in cutting the firewood. Suddenly, the saw jerks, and it is cutting the boy's hand instead of the wood.
"Then the boy saw all - since he was old enough to know." The narrator shifts from wishing the boy's youth was acknowledged to emphasizing that he was doing adult work and had to accept adult consequences. "though a child at heart - He saw all was spoiled. "Don't let him cut my hand off - the doctor, when he comes." The injured boy understands the severity of the situation, even in his youth, and begs his sister to prevent the inevitable from happening.
What the boy thought was going to occur (the amputation of his injured hand) doesn't happen, however. Those present are startled, horrified, helpless as
And then - the watcher at his pulse took a fright. No one believed. They listened to his heart. Little - less - nothing! - and that ended it.
And the poem ends with short, matter of fact phrases telling of the carrying-on of life by the survivors.