"Someone piped up and said this was not true, and that fuel pressure is solely governed by the fuel level in the tank, not by where it's drawn from, so that explanation could not be true. As long as fuel was above the "normal" pickup, fuel pressure (at the carburetor) would be identical whether the petcock was on normal or reserve. This went on to the point someone did an "experiment" where they attached a clear hose to the petcock and secured it parallel to the fuel tank, noting that the level in the hose did not change whether the petcock was on normal or reserve (it was always level with the fuel level in the tank), and thus "proved" pressure does not change whether you are on normal or reserve. "
Assuming that end of the clear hose was open to the atmosphere, this test will always only indicate the level of fuel in the tank and proves nothing about the pressure of the fuel in the hose. To detect a pressure change, the end of the hose would need to be closed with a means of either measuring any change in pressure of the trapped air in the hose when the petcock is switched to reserve, or observing any rise in the fuel level in the hose which would correspond to an increase in the pressure of the fuel exiting the petcock.