Exhalation while holding breath
The exact causes of the discomfort relief  after the breath-hold breakpoint  is unknown yet.

It has been stated that breath "pacemaker" continue to work independently of voluntary breath holding. Thus, while breath holding, the breath center is trying to activate the diaphragm and the afferent feedback from the diaphragm normally causes the feeling of discomfort. Thus, upon starting breathing again, i.e. actual diaphragm movements, the discomfort will diminish and the subject will feel relief.

Breath-holding and its breakpoint. Experimental Physiology Volume 91, Issue 1, January 2006.

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