Billysugger:
If both pin measurements are equal, then there is no net charge transfer between the electrodes, and no net electrolytic erosion.
I had guessed that with a "normal" measurement one of the two electrodes erodes,
but by alternating you achieve that both electrodes erode by the same amount ( and thus it's harder to see the effect )
Usually, you can provide a "sacrificial anode" to protect the other.