Basically what case fault geometry is is a limiting factor in how low you can set cracking effort on a regulator and have it stay stable in all positions. As I suspect you know, its the maximum difference in depth between the diaphragm and exhaust valve. In traditional (like barrel poppet) style regs, this is usually in horizontal, face down position. Lets assume for an example you have a reg in which the exhaust valve is 1.5" shallower than the diaphragm in that position. Since the exhaust valve limits the air pressure in the reg case, it will only hold shut when the pressure in the case is equal to or less than the ambient pressure at it's depth. If that's 1.5" less than where the diaphragm meets the lever, you have to set the cracking effort to 1.5" of water (or more) or else the pressure drop in the reg case will allow the diaphragm to collapse and the reg starts flowing. In real world applications, I've been able to set my balanced/adjustables or G250s to very low cracking pressures out of water, only to have them start to flow a bit when diving face down.
The idea behind the coaxial diaphragm/exhaust valve is that the difference in depth is never more than a fraction of an inch, so the regs are more stable at very low cracking effort. I definitely have found this to be true. The difference between the pilot/air 1 and D series is that on the pilot/air 1, the edge of the diaphragm is the exhaust valve, so I guess the maximum difference would be the radius of the diaphragm, while with the D series the exhaust valve is centered, so it's slightly closer to the center of the diaphragm.
The thing about the reg savvy book is that the concept is definitely explained, and you can see how it was part of the coaxial design of the pilot/air 1/D series. But there are other factors that I'm sure contribute to stability. These would be friction in the exhaust valve, friction in the lever arrangement, venturi assist aerodynamics, and probably the orifice/seat/spring consistency. I tend to use my pilot for most of my dives, and it's been very stable, but it does breathe wetter when upside down than do my D series regs. This is true with all the pilots and air 1s that I've tried. The only explanation I can think of is that the diaphragm must let a bit more water in when it does double-duty as an exhaust valve than does the smaller coaxial exhaust valve on the D series.
Anyhow, I bet you'll be able to test these ideas out for yourself in a very short time!
