It doesn't. The actual NASA test got micro-newton level forces on both the control and test rigs. The second vacuum test had essentially the same result, with the EmDrive, Cannae drive, and null Cannae all producing thrust (which is sloppy, there should have been another control). There's no such thing as a "virtual plasma" to push against, and with a closed cavity even if there was it'd violate the conservation of momentum.
tl;dr The few tests done have serious flaws, and if it works it breaks a great deal of known physics (which have been established with repeated testing).
It doesn't. The actual NASA test got micro-newton level forces on both the control and test rigs. The second vacuum test had essentially the same result, with the EmDrive, Cannae drive, and null Cannae all producing thrust (which is sloppy, there should have been another control). There's no such thing as a "virtual plasma" to push against, and with a closed cavity even if there was it'd violate the conservation of momentum.
tl;dr The few tests done have serious flaws, and if it works it breaks a great deal of known physics (which have been established with repeated testing).