More restrictive inhale when face up with regulator

[Akimbo]

This is NOT it, not the reason. The reason is described by Superlyte27: The fact you need to fight the "up" force of air acting on the 2nd stage diaphragm. You basically need to pull it "down" ward to activate the orifice. Same reason you get freeflow when mouth piece is facing up and no water in the 2nd stage.

You are saying the same thing. The up force is the differential pressure.

 

[eelnoraa]

What I am saying is it is NOT because the diaphragm is above the chest/lung. It is because diaphragm is facing up. You can try to lay flat at the bottom of the pool, so diagram and lung are at the same depth, the breathing effort will still be high because diaphragm is facing up

 

[Pinecube]

Just curious, would there be a noticable difference between a balanced and overbalanced regulator in when face up?