You do..(though it's simple, you just sort of humm or go "aaaaah" to yourself, silently.. that keeps your airway open).

No innate reaction - over-inflation would require a positive pressure applied to the lungs - normally as we breathe it's at equilibrium. Your lungs aren't elastic at all - once they are at capacity, over-pressure will rupture them right quick, and you own't feel it coming (but you'll sure feel it happening)