The inertial compensator was a device that reduced the acceleration felt by masses (including human passengers) a vessel experienced when accelerating.
While the gravity generators of a starship were capable of reducing felt acceleration up to 50 Gs, this limited the starship's acceleration to perhaps 51 Gs. A compensator prevented the crew and delicate equipment like computers from being squashed beyond any possibility of recovery at higher acceleration. If the compensator failed while the ship was under significant acceleration, all the crew aboard would be killed instantly.
A compensator generated a field which turned a vessel's impeller wedge into an inertial sump, greatly reducing the effect of acceleration inside that field. A compensator's efficiency depended on two items; the volume enclosed in its field and the strength of the gravity wave in its sump. (HH1) Compensators could function in the absence of an impeller wedge, but at greatly reduced effectiveness; a 150 G acceleration could only be reduced to an apparent 5 Gs. (HH6)
Prior to the First Manticoran-Havenite War, ships accelerated at no more than 80% of maximum compensator effectiveness (except in emergencies), to avoid an accidental compensator failure. As the war progressed, both sides found that this margin was overly conservative, and began running at higher accelerations more often.
In the middle of the 17th Century PD, the inertial compensator of HMS Queen Elizabeth I was sabotaged by Havenite agents, leading to the death of everyone aboard, including Queen Solange. (HHA2.2: WPD)
In 1873 PD, the Grayson Space Navy reinvented the inertial compensator using their own design. It was bulkier than Manticoran designs, but was more efficient, permitting higher accelerations for a given perceived internal acceleration. (HH2)
By 1904 PD, the Jankowski Cartel adapted the Grayson design for use by the Royal Manticoran Navy, increasing the usable acceleration of all refitted and new construction ships. It was a considerable military advantage. (HH3, HH8)