I heard it uses a cone. If you had two cones opposed to each other, then if you shot microwaves into one and the second would act as a collector to channel the reflected energy (the reaction ) back to the main gun.
They cant just shoot microwaves in one direction, it just bounces everywhere with the probes they are using, omni, and the whole cavity is in feed back which means just a hot standing wave in the middle.. i don't get it.. but maybe thats what they are trying to do,
http://arc.aiaa.org/doi/pdf/10.2514/1.B36120