When I was going to one, he used one of these as well. I viewed it as a "clicking" device. Probably with a spring from one of those ball-point pens, it was designed to make a soft "click" noise when button was pressed.
Seemed placebo-ish to me...
But, I'm no Chiropractor.
It seems to help with me. I’ve been going to an upper cervical one for years and it has been worth every penny. I wished I had gone to one long before.