Why do boys in Afgahanistan during the winter of 1975 have gashes on their fingers? What is a tar?

Boys were preparing for a big kite race in the winter of 1975.  According to Amir, "the kite-fighting tournament was an old winter tradition in Afghanistan" (51). Beginning on page 50, there is a detailed description of Hassan and Amir's preparation for kite racing.  They prepare "tar," which is the Afghan word for string, although I am not sure whether this applied specifically to string for kites or string in general.  As I understand this section, the boys took ordinary string and coated it with a "mixture of ground glass and glue" (50).  This string preparation made the string move quickly through a boy's fingers as he let it out to race the kite because the glass eliminated a certain  amount of friction.  However, this glass coating cut into the boys' hands as they elevated the kites.  Amir says that by the time spring came, "every boy in Kabul bore telltale horizontal gashes on his fingers from a whole winter of fighting kites'" (50). 

If you have never flown a kite, you may not know how painful it is to have even ordinary string dragged over your palm, but glass-coated string would be particularly painful to endure.