Inertial Dampener

Inertial dampeners are a system used on almost all starships to counter the effects of rapid acceleration/deceleration of a starship by sustaining and absorbing the natural inertia of a vessel as it either moves through space or if it is under attack from another vessel.

A starship cannot jump to warp speed without inertial dampeners, as the rapid acceleration would smash the crew into the walls, killing them instantly. (VOY: "Tattoo")

Although the inertial dampeners are activated by default, the crew can enable/disable and configure the strength of them manually. If the inertial dampeners are disabled, they can be reactivated by resetting the inertial dampening field base at three-zero-zero and re-initializing the dampeners. (VOY: "The Cloud") This use of them can exaggerate the damage sustained by a starship in a battle as the ship rocks more violently when its inertial dampeners are disabled. (TNG: "The Chase", "Gambit, Part II")

The entire crew of a Jem'Hadar attack ship (while saving a Changeling that would later die) was killed apparently when their inertial damping system failed as the ship went into warp. (DS9: "The Ship")

On stardate 50482.2, a cascade virus previously planted in the computers of the USS Defiant by Maquis defector Michael Eddington wiped all systems aboard. Several days later, the inertial dampeners had not been fully repaired and, while the ship could reach warp 6, Chief O'Brien added, "Don't expect a smooth ride". (DS9: "For the Uniform")