Interesting. My first reaction when I read the post was that they were in some way using electromagnetic radiation as the "exhaust mass." This is a terribly inefficient way of obtaining thrust, since the efficiency goes down as the velocity is a smaller and smaller fraction of the "exhaust velocity."
I read the paper linked to, and followed the equations. It appears they've found a scheme to use unbalanced forces from electromagnetic radiation in a cavity.
The paper was several years old. I hope they publish more soon.