How does blood from the kite string accentuate the kite flying scene?

I'd say sacrifice if I had to boil it down to one word. The kite strings cut up his hands, but it's the price he pays to win the contest. Hassan sacrificed a lot of himself for Amir, and the rape is a big part of that. Getting your blood spilled is losing a part of yourself, but it's usually for a noble cause in the case of this novel.