Ok, a look at why I think huge power is not needed for the repulsive reaction.
The electrode hammering noted by Ampere and others is an extreme example of the repulsive reaction, and I am sure that effect can be scaled down to the point where the electrodes aren't killed instantly.
Here goes: Looking at the working arc-gap at the moment the voltage is rising, there is a growing electrical gradient between the electrodes.  The air has not yet ionized, no current yet flows.  The voltage builds and builds until the ionization/arcing begins.  Very quickly, the arc becomes a low-resistance pathway for enormous current flow, which heats the ionized air to high temperatures.  Flash, BANG!
But at what point in the above does the 'hammering' or repulsive effect occur?
My bet is on the slender bit between "voltage rising" and "arcing".
Think of the vacuum, will you please?  Won't someone think of the vacuum!
It has needs, it has values, it has electrical properties.  Huh?  Who would have though that empty space has electrical properties?  Is does, so there.
Vacuum permittivity, to be exact.
It is the capability of the vacuum to permit Electric field lines.  More or less, it sets a limit  on electric field density.  If I read the back of the package right, there should be a limit.  Volume, density, time, volts per unit...
Hmmm.
Tiny gap between rods, fast rise-time to 'stay ahead' of hot-arcing.
Is vacuum permittivity being violated?  Not like punching a hole in a dielectric, more like the way an optical fiber can rupture with a powerful laser pulse. [solitons]
What if tweaking vacuum permittivity caused there to be more vacuum?
[And at this point in the narrative, the Author is safely assumed to have lost his mind.  Thank you, and good night]
More vacuum, as in more space/time.
Simple version: electrical field tension builds until initial conduction.  Electrical current 'pushes' space/time out of the way, until resistance of arc drops voltage gradient below some threshold.
[That should stir them up back home!  I need sleep.]
Less Simple version: Same as above, but the permittivity violation results in extra space/time being created to accomodate the excess field lines.  Once the field lines are gone [voltage gradient drops as resistance drops], the extra space/time ceases to be, the 'shove' exerted by the space/time bubble stops, but there is no rebound. [weak. needs work.  Think, man, think!  Saknussemm made it this far with a Q-tip and a 17 ounces of brass dust! ]

Why low-power threshold is important!
If a low-end for the reaction is found, say only a few milli-Newtons, this is good!
A low-power system will not eat the electrodes, and it can be run at high cyclic rates.
Five thousands pulses a second,  0.005 newtons each, hey, propulsion!

Personal aside: I have worked/played with electrical and electronic stuff most of my life.
I can glance at a schematic, and 'see' the AC and DC pathways.  Antenna design is like music to me.
So I have to say this:  I do not always like the implications of this work.
Persistent doubts remain.
Are the researchers all 'legit'?  Are claims being inflated?  Are 'they' keeping the truth from us? [tin foiling!]
No, the doubts I have are simple and sound: Laws of physics,  Conservation of Energy. 
TANSTAAFL
There Aint No Such Thing As A Free Lunch.
I am excited at the possibilities, but ground-down by my attachment to reality.
Need a new reality.