Impulse engines on star ships, based on placement, appear to be reaction engines that propel the vessel by projecting energy on a course opposite of the desired direction of motion.
This is backed up by Memory-Alpha which states:
In Federation starships, the impulse drive was essentially an augmented fusion rocket ... a vectored thrust nozzle to direct the plasma exhaust.
If this is the case then, why do all the star ships only have exhaust ports on the "rear" of the ship? In order to slow down or stop the ship would need to apply a change in velocity opposite to the current direction of motion. Without forward facing exhaust nozzles the ship would need to turn around to slow down, but this is never seen in any of the shows or movies (that I can recall anyway).
The ships do have maneuvering thrusters, but they always seem to be used for low speed maneuvering, and it is not implied that they have the same output as the impulse engines. Using them to stop a star ship moving at full impulse would probably take a very long time. Additionally, there is not any heat scoring on the ship hulls around the impulse engines, which might have suggested exhaust deflectors for braking action.
It would seem, therefore, that there is another technology in place to slow down a star ship at impulse speed, but as far as I can remember such a thing is never mentioned.
Is this ever mentioned in any canon source, regarding how a ship at impulse slows down?