Why do many girls wear black thread around their ankle. But anyway they call attention of others what is that it drives to tie it.?

I should say everybody should start wearing this! I had very good experince with using it. I am a girl with age @ 25. Wearing the black thread around ankle gives boost in confidance. Initially i was also hesitating to wear this. its really good to wear it. You should not wear it tightly so that it cause any hinderance to blood transfer, rather it should be lying very loosely. now i encorage every friend of mine to do so and they also observed the same.