"Catapult a vessel across space in the time it takes to say 'Catapult a vessel across space'."
The power core of the catapult generated a massive graviton surge that locked onto a ship through an array of projectors. The surge sent the ship hurtling into null space for some hours. Upon emerging back to normal space, the vessel had jumped many hundreds or even thousands of light years. The distance was determined by the graviton field strength.
In 2376, USS Voyager discovered a stranded alien starship, commanded by Tash. Their crew had built such a device as a means of getting home in a shorter period of time than by warp drive. The first test had destabilized the tetryon reactions of the tetryon reactor used as the power core, causing it to leak bursts of epsilon radiation. Voyager used graviton pulses fired from the deflector dish to stabilize the reactions.
Having helped Tash's starship to successfully use the catapult to get home, Tash sent recommendations for deflector shield adjustments to the Voyager so they would not experience similar system overloads during mid-flight. Following the enhancements to the shield variance, the crew of the Voyager used this device to travel across thirty sectors in less than an hour, taking three years off their journey home.
Seven of Nine theorized that the catapult technology may have been derived from the displacement wave technology used by the Caretaker to bring ships to the Delta Quadrant. Both the catapult and the Caretaker's array were powered by tetryon reactors, the only two ever encountered as of 2376.
Background information Edit
The script described this structure, which it offhandedly called a "space catapult", as "a strange piece of technology. A vast portal-like structure that looks cobbled together with sections of different design and alloys of different colors – as though the pieces came from several alien sources."
The design of the catapult station was later used as a Cardassian shipyard in the video game Star Trek: Armada II.