The Diametric Drive is a form of space drive which functions by creating an asymmetric field around itself by the juxtaposition of diametrically opposed field sources across the vehicle, hence the name. The side of the field which creates more force on the spacecraft will accelerate the spacecraft in the direction of the force. The most common form of this drive in sci-fi literature employs gravitational potential, but creating a gradient in electromagnetism is another possibility. One possible method for creating such fields is through the use of negative mass*, but it is unknown whether or not sufficient quantities of negative mass can be created and handled.
An illustration of the space-time distortions generated by a Diametric Drive can be found at http://www.geosoc.org/schools/adult/docs/Image21.gif
*see Negative Mass Propulsion
Information on space drives: http://www.grc.nasa.gov/WWW/PAO/html/warp/ideachev.htm