The inertial damping field (IDF), or inertial dampening field (IDF), is the forcefield that is especially designed to compensate for acceleration forces within a starship whenever the vehicle changes speed or flight direction. Several field-manipulation devices, called inertial dampers or inertial dampeners, are distributed throughout USS Voyager (and other Federation starships) to generate an IDF. During heavy attacks, the IDF can be temporarily unbalanced, often resulting in Voyager shaking violently with the impact, but the strength and flexibility of the IDF system reduces potentially dangerous forces to a minimum. On the approach to planetary landing, during the blue alert status that is called for such a procedure, Voyager's IDF is initially increased to maximum in order to cope with the huge gravitational stresses added to the motion forces. Once Voyager touches down in the designated landing zone on the planet surface, the IDF can be reduced to match the gravitational conditions of the planet, or even taken offline as the forces of motion generated in space travel are of course not present. Due to Voyager's ability to take off and to land, both the IDF (and structural integrity field (SIF)) systems carry specific enhancements to aid this procedure.

Inertial dampers are mentioned in several episodes. Examples:

In 2371, after re-entering the lifeform which they had previously thought to be a nebula in order to try and repair the damage wrought by the ship's phasers, Kim reports that they are "being hit by some kind of multi-polar charges". Tuvok says that "the polarity of these charges rotating so fast the shields cannot compensate", the rear driver coil assembly goes, the ship's inertial dampers go offline and the ship goes into freefall. Janeway orders Torres to carry out emergency shutdown of all thrusters, which means venting valuable deuterium fuel into space. Janeway also orders Paris to reset the IDF baseline at three-zero-zero and reinitialise. Paris does so, the field processors come online and inertial damping is restored and thus the ship is stabilised. [#6 The Cloud].

In 2373 the alien called Marayna throws U.S.S. Voyager around inside an inversion nebula. The shields are holding but the inertial dampers are offline, which is why the crew are being hurled around a lot. [Season 3: Alter Ego].

In an unrealised timeline of 2390, U.S.S. Voyager tries to travel back to the Alpha Quadrant using quantum slipstream drive. The slipstream collapses due to the phase variance. U.S.S. Voyager's hull starts to buckle and inertial dampers are offline. It is because the inertial dampers are offline that everyone inside the ship is being hurled about. Paris takes the ship towards an L-class planet 9 million kilometres away. Paris reports that they are only a few parsecs from the Alpha Quadrant. "Not exactly how I wanted to cross the finish line," mutters Janeway. The ship crashes on the ice-planet, killing all aboard. [Season 5: Timeless].

In an unrealised timeline in 2376 Chakotay shakes U.S.S. Voyager free of a Vidiian ship's clamp by reversing the hull polarity, which loosens the Vidiian ship's hold of Voyager a bit. Chakotay orders: "Re-route the deflector. Have it discharge anti-graviton pulses along the hull. Engage the inertial dampers. Get a controlled pitch going." It works and, to escape the Vidiian ship's tractor beam USS Voyager reverses thrusters at full power. [Season 6: Fury].

In 2377 a gravimetric surge is overloading the inertial dampeners, the prelude to U.S.S. Voyager being shattered into numerous different timeframes [Season 7: Shattered].

Inertial dampers were invented by Star Trek's writers primarily in response to valid criticisms that the acceleration and decelerations performed by the Enterprise would crush the crew unless there was some kind of heavy-duty protection. Source: Encylopaedia