OK -- I'm not an EFIE expert either, but I think I know what is going on. I assume you're using a FuelSaver-MPG analog narrow band EFIE.
If you put your meter between the two test probe ports on the EFIE, you are reading the voltage that you are adding to the O2 signal. Normally that reading should be between 250 - 350 mili-volts.
If you put your meter between the black port and ground, you will read the voltage coming from the O2 sensor. On MOST cars this will rapidly fluctuate between 0 and 1 volt. If you put your meter between the red test port and ground, your meter will read the corrected signal going to the computer --- the reading from the black port added to the reading from the red-to-black port.
As I said, MOST cars use a signal voltage of between 0 and 1 volts. SOME DOGES use a higher signal voltage ranging up to 2.5 volts.
If the meter reading between the black port and ground goes above 1 volt, then you have one of the odd-balls, so you will need to put that switch in the off position.
I hope I've cleared this up a little rather than just muddying the water further
Steve
1991 Plymouth Acclaim 3L V6.
1 dry cells with nineteen 6"x8" 316L ss plates, driven by constant current PWM set at 35 amps (13.3V at PWM). 28% KOH electrolyte. Total measured output 2.5 lpm. Mileage went from 18 to 26 mpg, all city driving (44% increase). EFIE set at .370 and I still need to play with ignition timing.