Its completely unconscious, when I was younger I would tense my neck really tight and it would make my head go back and forth. It was just me trying my best to get the word out. As Ive got older, I have pretty much stopped the involuntary movements by accepting that Im going to stutter, and not trying to force the word out. Different people have different 'ticks', some people flap their hands, stomp their feet, wobble their head ect. As you said, for some people its a physiological tool to help get the word out.