When you have a friend that is as corrosive as acid and mean as a snake but they like you unconditionally and do your dirty work on anyone beneath the two of you at a moments notice. Usually in a workplace. The string reference is in recognition they could turn on you in a second and burn you to a crisp.
I love my friend Lisa, she’s my dragon on a string..last week I got Barry fired for slacking off because he pissed me off. We laughed our asses off because he cried at his termination appointment.