Roger Shawyer, SPR Ltd, invented the EMdrive and recently gave an EMdrive presentation to the UK Defence Academy Shrivenham.

EmDrive is the first true Propellantless Propulsion technology.
High frequency electrical energy is directly converted to thrust.
It is a resonant microwave cavity, shaped to obtain different group velocities at each end, and thus achieve a force difference as the EM wave reflects off each end plate.

EmDrive is not a reactionless thruster, it is simply a new class of electrical machine.

Conventional microwave technology limits the maximum Q of resonators to around 50,000, giving a specific thrust of 200 mN/kW at 3 km/s. This technology will be utilised in first generation engines where typical applications will be the orbital transfer and orbit maintenance of communication satellites and the primary propulsion for science missions.