Overall, the poem has a very smooth, uninterrupted flow to it, but there are moments where its continuity is broken, drawing your attention to a phrase, which enhances the drama of the poem itself.
One such time is in line 18; in this line, the boy's sister has just called out supper, and the saw had "leapt" at the word. Frost describes how hand and saw met--so far, not much of an a abrupt break in the phrasing. But then in line 18, at the very end, Frost throws in an uncharacteristic short sentence. Most of the sentences were long, flowing, but line 18 is "But the hand!" Having such a short sentence there, tacked onto the very end of a line, draws the eyes to it, and makes the moment dramatic. We are alerted to the danger to the hand, we are concerned, we are thrown for a loop and startled by the brief declaration. And, it is a good moment to do it in, as his hand is indeed severely injured.
Another moment is at the beginning of line 27, when Frost declares, "So." This is after describing how the boy wanted to keep the hand, and how the doctor couldn't keep it. Using such a short phrase, the rather indifferent and ambiguous "so," is a way that Frost is able to relate both the people's indifference to the event, and the vast amount of things that COULD be said in that moment. He could have gone on and on about the tragedy, the despair, the fine details of the decision, but, in true modernistic style, he minimizes the tragedy, leaving the reader to feel it all, with as few words as possible. Then, look to line 32 as the boy dies; Frost says of his heartbeat, "Little--less--gone!" This accurately emphasizes the boy's death, and how awful it truly is.
I hope that those thoughts helped a bit; good luck!