I agree with you that it's odd that I cannot move my tongue when it's so relaxed. However 99% of people who stutter have this compulsive response. This is how stutterers block or prolong (core of stuttering) by not moving articulators while speaking. This compulsive response is also the reason that fluency shaping techniques do not cure 'a locked tongue' because it's not physical issue, it's actually a mental issue we people who stutter perceive a reason to justify or anticipate compulsion. Hence my question: how we can learn to move articulators without reason, to be more impulsive?