Inertia-suppression Machinery permitted the alteration of a ship's inertial mass and was primarily used to allow high-g acceleration for interstellar travel. It worked by manipulating the fluctuations and turmoil in quantum foam that caused momentum and inertia -- similar to the concept of the Higgs Boson particle. The device had four settings, or "states". State one was an increase in inertia, which had potential applications in stasis and defense technologies -- though humanity was only ever to achieve it on the microscopic scale. State two allowed for suppression of inertia to varying degrees, and was the standard function of the machinery. State three was a total suppression of mass, in that all matter would become photonic and would have to travel the speed-of-light; however, that state was only every expressed as a mathematical concept and never achieved in practice. State Four transitioned matter within its field from tardyonic to tachyonic, making it theoretically have no choice but to travel faster than c; while it was attempted several times -- both in labs and in interstellar space -- such a state was never successfully achieved.
During one of humanity's first contacts with an alien species a man named Sky Haussmann witnessed the use of inertial mass manipulation, but destroyed the vessel responsible rather than risk it falling into the hands of his adversaries. Centuries later, that same man met another grub, and resolved to do better. He used his organization to attack the one who held it captive, The Mademoiselle. Unfortunately he acted too late: the alien was dead, and its technical secrets had already leaked beyond Chasm City.
However, H, as he was then known, began hiring researchers to study what technology was left, including delving into the science of inertia-suppression. Eventually his efforts matured into a practical device which was outfitted onto his in-system ships, and finally into the former-Ultra lighthugger Zodiacal Light.
The Conjoiners, the only other human faction with access to the details of inertia-suppression, also created functioning machinery, and, after a successful test on the Nightshade, outfitted their evacuation fleet with it.
Perhaps inevitably, however, the technology was pushed beyond what it was capable of safely achieving. Skade, in an attempt to outrun the Zodiacal Light to the cache weapons, tried to modify the technology -- with the help of Exordium -- to allow faster than light travel. She did it via a state transition from tardyonic to tachyonic, or from transitioning to state four straight from state two, bypassing state three's practical impossibilities; her efforts were spectacularly unsuccessful, not only crippling Nightshade but also removing two conjoined individuals caught in the transition bubbles from all recent history.
It was a side-effect encounter by H as well, though his encounter was in a laboratory. Only those very close to the area of effect would remember a person who was erased or "unravelled" backwards in time, with recorded history indicating a much earlier death of that person. Because the victims -- both the deleted and nearby observers -- are the only thing that remain from the time line where the victims began, people suddenly panicking and searching for a 'colleague' who died years before -- in the new timeframe -- could be the only sign of an accident.
The Inhibitors made reference to a similar effect erasing entire species from history. As such, neither they nor any other species in the galaxy with the technology to build inertia-suppression machinery ever use them to travel faster than light.
- Though never confirmed, it is mentioned that the technology may have been linked to the alien tower in Diamond Dogs, Turquoise Days.