Sort of like a rail-gun without the rail.
Shawyer claims that his drive does not violate conservation of momentum and is not reactionless.[9] Shawyer has posted an updated theory paper (version 9.4) for the EmDrive.[17] Shawyer's paper includes the fundamental assertion underlying the theory: "[t]his force difference is supported by inspection of the classical Lorentz force equation F = q(E + νB). (1) If ν is replaced with the group velocity νg of the electromagnetic wave, then equation 1 illustrates that if vg1 is greater than vg2, then Fg1 should be expected to be greater than Fg2."
This statement makes two assumptions which Shawyer does not substantiate and which may explain the discrepancy between Shawyer's predictions and those of conventional physics. First, Shawyer assumes that radiation pressure is the result of the Lorentz force acting on charged particles in the reflecting material. This is analyzed by Rothman and Boughn[18] who point out that the standard theory of radiation pressure is somewhat more complicated than the simplified analysis suggests.
Second, Shawyer asserts that quantum energy is transferred at the group velocity, and thus momentum of the photon and the consequent radiation pressure must vary with group velocity. Photon momentum varies with phase velocity. Group velocity measures the rate of propagation of information. The phase velocity is constant throughout the frustum resonator, consequently radiation pressure would not be expected to produce unbalanced forces.
http://en.wikipedia.org/wiki/EmDrive