I think we are basically both right.

Doing outwardly aggressive things like yelling and screaming does release adrenaline, and that will basically give you increased pain resistance, strength, better/faster reflexes.

It also raises your heart rate and thus increases blood flow and oxygenation levels.

I don't know that moving your diaphragm substantially would... directly somehow move your blood more or faster, but it would trigger a bunch of other hormonal reinforcing loops that would lead to your heart pumping faster/harder, i think, if you're substantially flexing it in a way that you typically do not.