As per the TNG technical manual, the main propulsive force is provided by a collection of fusion engines (on the Galaxy-class, two on the main saucer, one on the engineering section) which produce a massive amount of thrust, as well as supplementary power in case of Warp Core failure. However, from the Ambassador-class onwards, larger Federation Starships have used a space-time driver coil (similar to the warp field coils in a Warp Nacelle) to generate a low-level (slower-than-light) distortion field, allowing the thrust to propel the vesselās mass more effectively (at this point, weāre otherwise talking about using fusion engines to accelerate a multi-million tonne vessel to large fractions of c.)
During emergency situations, later-generation impulse engines are designed to allow minute amounts of antimatter to be injected into the impulse reaction chamber to provide a short burst of additional power or thrust.
In standard operation, Impulse is limited to an operational top speed of 0.25c, though this is more to avoid relativistic effects than because the engines canāt go faster (which warp travel circumvents) - a Galaxy-class vesselās impulse engines can reach velocities in excess of 0.75c, with all three engines at full power, but this is far from routine.