A superluminal drive, rather than a subluminal engine, is the technology that allows modern spacecraft to move at speeds significantly beyond the speed of light.
Unlike subluminal drives, which come in forms and encompass a wide range of approaches to providing propulsion at sublight speeds, there is only one technology capable of “breaking the light barrier:” the Artificial Tachyon Induction Field, produced by an Artificial Tachyon Induction Drive, or ATID.
The ATID has existed for as long as faster than light travel has been possible. When active, it creates a field of energy that alters the nature of space within it, creating a discontinuity between the contents of the shell and the rest of the universe. Normal space/time responds to this discontinuity by crushing it down to nearly the size of an electron, and in the process it takes on many of the qualities of a tachyon.
In practical terms, the ATID takes a ship's mass and converts it into an 'imaginary' mass (i.e., if you square the mass, the number will be negative). The 'higher' the negative mass, the faster a ship will travel above and beyond the speed of light. This means, as a general rule, that larger ships can go farther and faster than smaller ships. Current technology allows ships to travel at a maximum of 200-500 times the speed of light. There is currently no consensus in the scientific community as to whether there is a theoretical limit to speeds an ATID can reach.
This is, of course, a very imperfect and imprecise description of the actual process. Is a ship really crushed down to the size of an electron? No: if that were indeed the case then everyone on board the ship would be instantly destroyed, as would the ship itself. It is more accurate to say that the ship and its occupants exist in an artificially generated universe that is anchored to our universe at a point that is roughly the size of an electron, but even that explanation takes liberties with physics in ways that will make a trained physicist object, usually loudly and with great exasperation. But while this explanation takes liberties with the science behind the device, the terminology commonly used by crew on spacecraft takes far more: an ATID is usually referred to as a “Tachyon Drive,” the process of activating an ATID is referred to as “jumping to tach” or “folding” or “entering foldspace.” All of these terms have been known to make physicists genuinely angry.
It is important to note that there are significant environmental restrictions to an ATID's ability to operate: specifically, an ATIF cannot form in the presence of any substantial gravity well. This means that spacecraft must rely on their subluminal drives until they have travelled out of a system before activating the ATID. Also, a gravity spike while an ATID is active can cause the field to collapse, and the results of such a field collapse are generally considered catastrophic to the crew and passengers.
Taken from An Informal Guide to Known Space, Edition XVI, McPherson/Sovitt Publications.