The driver coil is a stage in 24th century impulse drive, using energy from the impulse fusion reactors to envelop the starship in a low-energy subspace field intended to lighten the relative mass of the vessel it encompasses. This significantly reduces the mass burden on the impulse drive, allowing for unprecedented rates of acceleration.
In 2370 in the mirror universe, the impulse driver coil on Benjamin Sisko's starship was offline. (DS9: "Crossover")
In 2373, Miles O'Brien ordered the driver coils of the USS Defiant to be configured for impulse speed. (DS9: "For the Uniform")