Mass lightening
Mass lightening is a technology used by many races in Star Trek. The effect of this technology is to reduce the effective mass and inertia of a large object like a starship so it can be accelerated quickly with a relatively small amount of thrust.
How it Works
Mass lightening technology is fictional, and its fictional principles have never been clearly explained in any Star Trek series or movie.
The technology is known to involve subspace field effects, so a common hypothesis is that the technology somehow shifts the ship's mass from normal space to subspace, where it has no inertia. A rather small amount of thrust is sufficient to rapidly accelerate the mass that remains in normal space.
A consequence of using mass lightening technology is that Star Trek ships are highly susceptible to subspace phenomena, like the subspace shockwave from the Praxis explosion[1].