If you want to test this yourself, try taking the deepest breath you can, then feel your throat. It will be tight, constricted. Many stutterers attempt to speak like this, thinking lungs full of air will help blast out the words. This results in you struggling against the muscles in your throat which exacerbates the issue. Now, breath through your diaphragm slowly, allowing the muscle to slowly rise above your stomach. Begin exhaling as soon as you feel you have a "comfortable" full breath of air. Do not breath in all the way. Your throat - and vocal cords - will stay relaxed 100% of the time, allowing you to generate speech. Also - watch the tensions in your articulation areas (jaw, tongue, lips). Keep these relaxed - a common thing stutterers do is clench their jaw, press lips too tightly together on "M" sounds, or push their tongue to the roof of their mouth while breathing (I do this one and am currently fighting my instincts to readjust my tongue to a relaxed position at the bottom of my mouth).