An impulse drive is a sub-light propulsion system that uses high energy plasma from one or more fusion reactors to propel a vessel. Unlike their smaller Ion Drive cousins the Impulse Drive is designed as a separate propulsion system from a vessels matter/antimatter reactor. This allows the drive to operate even a Starship's primary Warp Drive or matter/antimatter reactor is not functioning.
Design Scale
On smaller vessels (fighters and shuttles) Impulse Drive can be powered directly from a single Matter/antimatter reactor, though this does stop the system from being used as a backup power source. On large scale vessels, banks of fusion reactors will work in concert producing high energy plasma for large impulse drives, each reactor producing a portion of the thrust.
Operation
One or more fusion reactors are linked together in series. They generate high energy plasma streams which are split. One stream is then shunted through a magnetic accelerator to increase the particle velocity while the other is directed as a field coil assembly that serves to warp spacetime around the vessel. The primary stream, after being accelerated, is pushed through a vectored exhaust nozzle. The force of the particle exhaust then moves the ship similar to a fusion rocket.
Speeds
The impulse drive system is used to provide acceleration that approaches relativistic speeds. The actual maximum speed of an impulse drive can vary from ship to ship but most drives have a top end full impulse power between 0.1c and 0.25c.
Relativistic Concerns
In the thrust plasma stream there are a series of sub-space field coils which alter the spacetime geometry around the vessel to account for a good portion of the relativistic concerns are high impulse speeds. This effect is not perfect but the altering of the spacetime geometry around the vessel counter-acts the relativistic effects up to about 95-99.73% of the time dilation effects at such velocities. This effect is one of the primary distinguishing factors between an impulse and ion drive system.
Faster than Light
In the mid 22nd century it is known that Romulans used something very similar to an impulse drive system powered by an artificial quantum singularity at their engine core for warp propulsion. Other instances where vessels which have been severely damaged have used their impulse drive to travel faster than light also exist, but it is very costly on fuel and not recommended. By the 23rd century Romulans were using a more accepted warp drive design.
Power
Fusion reactors that feed the impulse drive system can also be used for power. In this case the plasma stream from the reactors is redirected into the power grid of a vessel providing an alternate power source if the main systems are inoperative.