Reversing the Impulse EnginesTo accelerate a Galaxy-class starship to 0.25c, the amount of energy needed (using E=0.5mv^2), is 1.265*10^25 Joules, or equal to 70,000 tonnes of matter and 70,000 tonnes of antimatter. And to accelerate the ship at 1000g (full impulse), the power needed is nearly 1.7 billion terawatts. Or power needed to accelerate the ship well past Warp 9.9.
According to the Star Trek: The Next Generation Technical Manual, page 75, it states that the driver coil assembly is similar to the warp engine nacelles that would perform a low-level continuum distortion without driving the vehicle across the warp threshold. In other words, the impulse engines drives the ship to the unrealistic speed of 0.25c at an unrealistic acceleration of 1000g using a miniature warp drive, not the Newton based thrust everyone else believes.
So if the ship is driven by space warps for sub-light speeds rather than action/reaction, then just like warp drive, the ship decelerates instantly by simply shutting down the impulse drive. And why are the impulse engines at the back? Plasma venting so the ship doesn’t run into the plasma, and Newton based thrust can come from transferring impulse power to the thrusters for parking orbit or where kinetic energy is needed.
Why not just use the warp drive for sub-light flights as well? On the Constitution class, Enterprise class, and all ships before the Ambassador class, they might have. But needed a lot of power just to keep the warp engines warmed up, and the more compact driver coil assembly was probably more feasible on an energy budget. How the Enterprise was able to achieve Warp 0.5 on "Star Trek: The Motion Picture", able to cross V’Ger with the shortened 2 AU in diameter in a few minutes. On TOS: “The Paradise Syndrome”, where the Enterprise’s warp drive was damaged beyond repair, and had to reach the planet using impulse drive (no DCA), while taking weeks to get back.
And if the space warp is always accelerating a ship forward, if there is an alteration in its course, then the space warp will push the ship to the new sense of forward and not the old. Being able for the ship to turn and pivot like a submarine. On the Voyager episode, “The Cloud”, it was mentioned that the thrusters have drift mode in them. Meaning that they may not be totally driven by Newton physics? Still have yet to figure out Star Trek: Destiny; Book 1 as to how the Columbia (NX-02) was able to accelerate to light speed and use time dilation to reach a planet light years away with their warp drive destroyed.
(Theory by Robert Heckadon)