When someone stutters, their brain is already producing too much tension in the muscles involved in speech including our diaphragm, which is where you felt the tension. If someone struggles against their stuttering, they add even more tension which can lead to the diaphragm to completely lock. If someone learns to relax instead of fighting against their stutter, they will be more fluent. Nothing about this is easy or simple but possible with the right technique and lots of practice.