If you think of mass as the deformation of spacetime, mass reaction engines push off the wake from expelling mass rapidly in one direction to go in the other direction.

The EM Drive is an energy reaction engine, rather than a mass reaction engine (if it works as described), so instead of pushing off wake, it uses energy itself to "deepen the fold", as it were, and increase speed (and mass, since the two are exchanged).

That's interesting. I've never considered that the very act of increasing the velocity of the EM Drive would actually increase its mass. Hrm.