(covers information from several alternate timelines)
The chrono deflector was a time travel-device invented by a Klingon scientist named Korath in the early-25th century. The device operated by generating a powerful tachyon pulse to form a temporal rift, which could then be directed to a specific time and place. Although able to generate a stable rift, the device also produced a large amount of tachyokinetic energy as a byproduct of its operation, posing the risk that it would burn itself out after a single use, prohibiting a return trip for the user.
In 2404, Admiral Kathryn Janeway made a deal with Korath to secure him a seat on the Klingon High Council in exchange for use of the chrono deflector. But when Korath demanded an ablative generator in addition, Janeway was forced to steal the deflector. With the help of Harry Kim, Janeway installed the deflector on her shuttlecraft and used it to travel to the Delta Quadrant, the year 2378. Encountering the USS Voyager, she instructed her counterpart to emit an anti-tachyon pulse towards the rift to seal it, to prevent Korath from pursuing her back in time. Admiral Janeway subsequently collaborated with the Voyager crew to change history and return the ship to the Alpha Quadrant sixteen years before it would have without her intervention. (VOY: "Endgame")