It's like walking on a 2 ft wide plank, suspended 50 ft high in the air.... You can easy walk on this plank if it was lying the ground, but your sub-conscious literally stops you if you try walking on the exact same plank suspended high up in the air. So basically, your sub-conscious is trying to save you from a potentially deadly situation. Same concept with stutter -- our sub-conscious perceives danger in feared situations (danger of getting ridiculed, perceived as being lesser than others, etc.), and does everything in it's power to stop us from speaking.