Inertial Damping System
This information can be found in Section 2.5 of the Star Trek: The Next Generation Technical Manual.
Operating in parallel with the structural integrity field (SIF) system is the inertial damping field system (IDF). This system generates a controlled series of variable-symmetry forcefields that serve to absorb the inertial forces of spaceflight which would otherwise cause fatal injury to the crew. The IDF is generated separately from the SIF, but is fed by a parallel series of waveguides that are then conducted through synthetic gravity plates.
The IDF operates by maintaining a low-level forcefield throughout the habitable volume of the spacecraft. This field averages 75 millicochranes with field differential limited to 5.26 nanocochranes/meter, per SFRA-standard 352.12 for crew exposure to subspace fields.
As acceleration effects are anticipated, this field is distorted along a vector diametrically opposed to the velocity change. The IDF thereby absorbs the inertial potential, which would otherwise have acted upon the crew.
There is a characteristic lag time for the shifting of IDF direction and intensity. This lag varies with the net acceleration involved, but averages 295 milliseconds for normal impulse maneuvers. Because IDF control is generally derived from Flight Controller data, normal course corrections can be anticipated so there is rarely any noticeable acceleration to the crew. Exceptions to this sometimes occur when power for IDF operations is restricted or when sudden maneuvers or other externally caused accelerations occur more rapidly than the system can respond.