Note: This article discusses material that was once considered canon but whose canonicity is now questionable due to contradiction by new sources.
Unlike an Imperial Warp-Drive, which allows a starship to fully enter and navigate through the Warp to achieve apparent superluminal speeds and reduced travel times in realspace, the T'au Ether Drive only allows a starship to make a brief "dive" into Warpspace.
While a T'au starship outfitted with an Ether Drive can still achieve apparent superluminal velocities from the perspective of realspace, interstellar travel using this technology is far slower than conventional Warp travel.
Despite the T'au's lack of psykers, and their lack of understanding of the true nature of the Empyrean, the Earth Caste have nevertheless developed an effective drive system that is sufficient to suit the needs of the rapidly expanding T'au Empire.
On the innermost of the world of T'au's seven moons, a routine geological survey discovered the remains of an alien vessel. The significance of the find did not disrupt T'au society as much as might have been expected.
T'au theorists had long reasoned that other intelligent life forms existed in the universe and the verification helped confirm the belief that there was a greater destiny awaiting them. No T'au commented on the sheer good fortune of finding the technology that they so desperately needed on their doorstep just when they needed it.
The T'au were able to duplicate the Warp-Drive of the alien ship but the initial test flights were disastrous. Achieving transition to the Warp required more than technology, it required psychically attuned minds; and the T'au race boasted no psykers.
Without them to guide the transition, no amount of power could breach the dimensional barriers. The best the T'au could do was make a partial transition, forcing themselves into the void that separated the Warp and realspace before they were hurled out again like a ball held under water then released.
Data gathered at great cost during the test flights was studied closely. Earth Caste scientists made the observation that the boundary between realspace and Warpspace was not a neat line. It was closer to being a turbulent ocean fomented by the tempestuous Warp tides below.
By carefully angling their "descent" toward the Warp and extending the field generated by the gravitic drive into a wing, shaped to hold the vessel down in the Empyrean tides, a T'au vessel could extend the duration of the dive considerably.
The speeds achieved in the "ascent" back to realspace were staggering and this coupled with the effect of the Warp on time and space ensured that the real distance covered by the dive was immense. Early tests lost several Drone ships because they inadvertently passed far beyond the sensor range of their recovery vessels.
The details were soon resolved. There was still a major constraint on this so-called "Ether Drive," for only the most powerful (and bulky) drives could sustain the gravitic wing throughout the dive and the power drain meant that considerable recharge time was needed between dives.
Also, in comparison to a starship that fully entered and navigated through the Warp using a conventional Warp-Drive, the pace of superluminal travel was very slow.
In comparison to the typical superluminal speeds achieved by Imperial voidships outfitted with Warp-Drives, the T'au Ether Drive was slower by a factor of five. The speed was consistent though, and did not expose the T'au to the myriad perils of the Warp, enabling the T'au species to expand beyond their home star system for the first time.
In earlier editions of Warhammer 40,000, all T'au starships used the Ether Drive, accounting for their slower rate of interstellar expansion and less technological sophistication with Warp-based technologies than the Imperium.
Since the introduction of the 6th Edition Codex: Tau T'au vessels have instead been propelled by the sublight ZFR Horizon Accelerator Engine. This drive does not translate the vessel into the Warp at all; instead it is capable of driving the T'au starship to velocities very close to the speed of light in realspace.
While this means that from an outside point of view a T'au vessel can take many Terran years to cross an interstellar distance, due to relativistic time dilation, its crew will experience a much shorter passage of time from their frame of reference.
- Battlefleet Gothic Armada, pg. 96