Hehe, do report back, I don't think I know myself!
From what little I do understand, there's some sort of rotating emitter arrangement under the flat bed on the bottom of the microwave. Panasonic say -
"In the case of Turntable style microwave ovens, the microwaves were emitted from a fixed antenna that is located on the side of the cavity. These emitted microwave will then bounce around the cavity. With microwave ovens that use flatbed technology, the microwaves are emitted from a rotating antenna located underneath the ceramic base of your microwave oven."
Now that's interesting, how do they work - is it a rotating waveguide? I see an hour or two of web 'research' ahead.