fair question! I can explain this very simple. The proces of coordination of the muscles are working perfectly smoothly by default for everyone regardless whether they stutter or not. HOWEVER, when the diaphragm makes uncoordinated movements and as a result the psg air flow is disrupted it is fysically impossible to speak fluently. But when you try to do so still this can result in so much tension in all of the muscles above the larnyx. Often when people are afraid the first thing they do is inhale, you cant speak on a inhaltion