It is an issue with the geometry of the second stage case and the relative locations of the exhaust valve and diaphragm.
With a conventiional second stage case design, in a normal face down position air will leak out the exhaust valve (which is about 1.0 to 1.3" higher in the water column than the center of the diaphragm) until the pressure inside the case is equal to the pressure at the exhaust valve (plus any resistance needed to slip past the exhaust valve).
So in the real world what happens when you stop inhaling between breaths is that diaphragm moves inward as the air leaks out the exhaust valve until the resistance of the lever stops the inward movement. This means that when equilibrium is reached, there is some pressure preloaded on the demand lever and it is there is that much less inhalation pressure required to further depress the lever and open the valve for the next breath.
In fact, if the inhalation effort on the reg is too light, the diaphragm will depress the lever enough to cause the valve to open slightly which results in a small but constant freeflow out the exhaust valve during the pause between breaths. This is why many conventional second stage designs cannot be practically set with inhalation efforts below about 1.0 to 1.3" unless they are equipped with an adjustment knob.
Now, when you are looking face up, the situation is reversed. The pressure at the exhaust valve is about 1.0 to 1.3" inches of water greater and the pressure inside the case stabilizes at this higher pressure. This pressure is then attempting to push the diaphragm outward (as the pressure is lower outside the diaphragm) and this pressure has to be overcome before the diaphragm can begin to depress the demand lever to initiate airflow for the next breath.
The result is that the inhalation effort is noticeably higher in the face up position with a conventional second stage design.
As indicated in the prior post, the Scubapro D400 breathes equally well in any position. This occurs because the exhaust valve is co-axially mounted in the center of the diaphragm and in addition the diaphragm is angled downward 45 degrees. This means that the maximum pressure differential is at worst only about .5 inches of water (the distance from the edge of the exhasut have to the center of the exhaust valve and diaphragm.) and that this worst case scenario only occurs when the diver is looking up at 45 degrees or looking down at a 135 degree angle. When looking straight up the differential is approx. .2 inches and in a normal swimming position it is .0 inches.
This means the poppet itself can be set to operate at inhaltion efforts as low as .6 inches of water and still be stable and that any increase in inhalation effort is limited to .2" (minus the initial resitance of the poppet). In practice there is virtually no percpetible change in inhaltion effort regardless of diver orientation.
Usually, when breathing in a face up position with a D400, the purge cover traps air in the "top" and will prevent water from leaking through the exhaust valve during exhalation. However, if you are angled off to the side while face up it is possible for the D400 to breath a bit wet as the water leaking into the case cannot run up hill out the exhaust valve mounted in the diaphragm.
The face up position is also a situation where it is very desirable to have an adjustable regulator. If the reg is properly tuned (so that it is on the edge of a freeflow, or even has a very slight freeflow when the knob is turned all the way out), you will have to turn the knob in a couple turns to prevent a freeflow in the worst case face down swimming positon to prevent a freeflow between breaths. But those couple of turns on the knob will then allow you to adjust the reg for normal breathing in a face up position. The pressure problem still exists, but the poppet will be on the edge of a freeflow already and will require very little inhalation to intiate air flow.
Double hose regs tend to force feed you in a face up situation, especially if you are swimming level in the water, as the pressure acting on the second stage diaphragm (located in the case attached to the tank valve) is a few to several inches below the mouthpiece where the air is trying to exit. It is not what I would call comfortable as you feel like the reg is trying to inflate you.
