George Dvorsky does a great job of breaking down the recent results concerning testing of an electromagnetic drive in a hard vacuum in this io9 article. The testing was originally conducted outside of a hard vacuum before this recent test. Dvorsky successfully outlines the challenges that scientists now face in trying to understand a drive system that should not be able to work given mankind’s current understanding of physics.
An EM drive supposedly creates propulsion without any type of propellant, such as rocket fuel, by exciting microwaves inside of a closed containment system to create electrical power. Last year, the same claim was made, but results revealed that the test was not conducted in a hard vacuum.
As a result, the thrust was thought to likely stem from thermal convection. With the recent test in a hard vacuum, there is now serious confusion about how scientists were able to measure thrust since a vacuum has always been perceived as not having anything to push against to create thrust — which is why propellant is used in rockets and other space craft. The article further outlines that this topic is being heavily discussed by amateurs and experts in the NASASpaceFlight.com forum.