The ultra-transwarp drive was the engine secretly developed through the X-Project to allow a ship to travel at faster than transwarp speeds. Housed within the engine were the two sub-functions of transwarp and warp capability. (Star Trek: Phoenix-X)
In 2374, Dawn Relic hijacked the Phoenix-X and forced it into a premature ultra-transwarp jump. The ship landed in the Gamma Quadrant, but the ultra-transwarp components were damaged beyond repair. As a result, the only capabilities the engine was allowed to produce were transwarp and warp. ("Crash Bandicoots")
Ten years later, Captain Cell discovered that the X-Project and the ultra-transwarp's components and designs were deliberately stolen from the alien Whale Probe by Section 31 in the late 23rd century. When both Section 31 and the USS Phoenix-X encountered the whale probe once more, in 2384, Section 31 used the opportunity to further steal more components. ("Forbidden Crossing, Parts I & II")
Brought with the additional components was the returned ability to bring functionality to the ultra-transwarp. The Phoenix-X was then sent on a mission to the distant Andromeda galaxy. Upon arrival, the ship was forced out of transit through the ultra-transwarp conduit due to rising radiation levels originating in Andromeda. Luckily, Armond and Kugo were able to neutralize the adverse effects to the conduit in order for the ship to return home.