Are you talking about more than blocking? Your body knows how to make sound, some of that sound may be disfluent but you've been conditioned that that's not acceptable so you avoid it at all cost, thus you find yourself in this battle where you feel you have to push because there's an opposing force keeping the words back. But the truth is there is no mystical force doing that, it's all you and within your control. You nervous system is conditioned react the way it does, but you can uncondition it as well.