For the purpose of this mod, you can think of a diode as erecting a barrier that is approx. 700 mV tall: @700 mV and below, no current flows, but everything above 700 mV passes over the barrier. (The "height" of the barrier, called the diode's "forward voltage" -- the minimum voltage required to turn the diode "on" and permit voltage and current to pass, varies a bit by type of diode). This applies to a "forward biased" diode - i.e., one where the anode is more positively charged than the cathode (banded end). Because the mod uses two "reverse paralleled" diodes, there is a forward-biased diode in each direction: to earth ground and from earth ground -- so it blocks sub-700 mV currents in both directions.
In reality, the "barrier" is sloped and some small amount of current makes it through when the applied voltage is less than 700 mV. See attached graph, and specifically the region to the right of the X,Y origin.
cheers, Derek
