ZFR Horizon Accelerator Engine
Following the First Sphere of Expansion, a geological survey uncovered the remains of an alien vessel on the innermost of T'au's moons. The T'au were able to reproduce the ship's Warp Drive, but lacked the necessary Psykers to breach into the Warp. Instead, they began using this warp drive in combination with their existing gravitic drive to partially transition, or "dive", their ships into the void between Materium and the Immaterium. Doing so would lead to the ship ascending back to the Materium at near-Light Speed; "like a ball held under water then released.".
Despite this, they were only 1/5th the speed of typical Imperial warp speeds, and the drives were bulk and used large amounts of power, needing considerable amounts of time between dives to recharge. However, this form of travel did not expose those aboard to the Warp, and travel at a consistent speed compared to the unpredictable effects of Imperial Warp travel.