Usually, a propulsion system generates thrust by blasting something in one direction, thereby causing a craft to move in the opposite direction. Rockets, for instance, shoot superheated gases out of nozzles in order to get off the ground and into space. Instead of blasting propellant in one direction, the EmDrive (Em stands for for "Electromagnetic") bounces microwaves around a cone-shaped chamber. There's no exhaust, so there doesn't appear to be anything that can generate thrust. That means that if the drive works, it might break Newton's third law. (Shawyer, for his part, says it doesn't.)
Despite that, NASA researchers witnessed the EmDrive generate small amounts of thrust in a lab. The system they tested produced 1.2 millinewtons of force per kilowatt of energy—100 times more than a solar sail, one of the only propellant-less spacecraft that's currently feasible. It should be said that the study is preliminary and needs confirmation, but if it's true, it could mean big things for technology. According to Shawyer, the EmDrive could be used not only in spacecraft, but as a way to power flying cars and provide energy for the entire planet. Learn more about this controversial technology in the videos below.