The gravimatrix was a unique power production facility on the planet Tigan. It harnessed the power of a micro-black hole, using the planet's geothermic energy to maintain the necessary containment field.
Whilst testing the gravimatrix a gravimetric pulse was generated which wiped the Tigan central computer. The data had to be re-input by hand and the politicians of the time took the opportunity to make a few adjustments to the planet's history. From then on successive regimes used the same system to make alterations, at first on an infrequent basis; ten times in the two centuries leading up the 2360s decade but eventually more frequently; six times in the decade leading up to 2364. The practice was brought to an end when Edic a man who’s family had long been involved with the conspiracy, saw the wrong that was being done and rebelled against his fellow conspirators. (TNG - The Space Between comic: "History Lesson")