Star Trek Expanded Universe

The gravity induction drive was a type of starship propulsion utilised by the Taurhai Unity. The drive created a subspace funnel, in essence a micro-wormhole, which the vessel that falls through allowing up to a maximum jump of 3 light years in mere seconds. Once a vessel was in its funnel it was undetectable, allowing Taurhai vessels to seemingly appear out of nowhere with no warning. (Ship Recognition Manual, Volume 5: Starships of the Romulan Star Empire)