Why does Justin tap his fiddle case when he confronts Julian and his friends about their treatment of Jack?

Julian taps his fiddle case for effect when he warns the bullies not to mess with Jack:

yo, listen up. don't mess with jack," i say really slowly, gritting my teeth, my voice all clint eastwood tough-guy. "mess with him again and you will be very, very sorry." And then i tap my fiddle case for effect. "got it?"