Of course, nobody is getting a free lunch here. "Although the proposed engine will consume energy for manipulation of the particles, the propulsion will occur without any loss of mass," says Feigel. He even suggests, with masterful understatement, that this might have practical implications.
So basically, you can create thrust by interacting with virtual particles that don't actually exist. It's not quite a zero-point generator, but it still relies on pushing against fiction.
I guess it works basically the same way as turning electricity into thrust by pushing against a planetary magnetosphere, but without the planet?