Subspace field generator was a technological component on Federation starships used for generating a warp field. Engineers aboard these starships had access to technical manuals for these generators, which were stored in the Engineering Systems Database. For the Galaxy-class USS Enterprise-D, Doctor Leah Brahms wrote the operating protocol for these generators.
Prior to 2365, Dr. Brahms, as Design Engineer, supervised the implementation of improved generators onto the Enterprise and its sister ship, the USS Yamato, when both ships were undergoing construction at the Utopia Planitia Fleet Yards, Mars. (TNG: "Booby Trap", display graphic)
These components were capable of decreasing an object's mass, literally making it lighter. In 2375, Jack, Patrick, and Lauren, three genetically-engineered psychiatric patients, conceived of a plan whereby, through the use of a massive amount of subspace field generators, they could decrease the mass of the universe, thereby changing the cosmological constant and averting the end of existence. (DS9: "Chrysalis")