What's causing the effect of salt in voltage arcs?

SPECULATION:

"Does the salt decompose during the process?"

I suspect that it does. The salt ionizes easily and the ions would migrate under the influence of the electric field. In so doing, they will further ionize the air they traverse, creating a stream of charge carriers. As the electrodes are pulled further apart, the energy available for ionization becomes smaller (the ions gain less energy per unit length as the voltage is constant but the distance increases, so the electric field strength goes down).

At a certain distance, the ions lose more energy during each collision than they gain during the acceleration due to the electric field - and then the arc extinguishes. So the arc is a result of the balance between ionization and recombination. The thing about having an electrolyte on one electrode only: there is no charged particle to recombine with. That probably really helps. If I'm right, then putting electrolyte on both electrodes would be less effective. Now I'm curious…

Incidentally - it sounded like this was an AC arc (there was a distinct low frequency buzzing) - but that doesn't jive with the above explanation unless the ions have a really long lifetime...