They're called cerol in Brazil, and used for kite battles. The string is run through glue, then crushed glass shards. The game is to cut the other kites string with yours.
However, it's often motorcycle and scooter riders that catch the string when the kite falls. Last one I saw took the head right off at the neck, still wearing the helmet. This guy is lucky to be alive, although not quite as lucky as he'd want.
