This article is from the Star Trek Tech FAQ, by Joshua Bell inexorabletash@hotmail.com with numerous contributions by others.
Kirk and Sulu use a combination of warp drive and Impulse to break
free of the First Federation pilot craft. The combination of a tractor
beam, impulse drive, and warp drive would be very strange, and many
explanations come to mind, such as the warp field causing the tractor
effect to "slip" away, while the impulse provides propulsion, or the
impulse fighting the tractor beam inertially while the warp drive
provides propulsion, etc.
In The Voyage Home, for the trip back to the future, thrusters are
used by Spock to get the last burst of speed just before entering time
warp. Also, during both trips, the ship is brought out of time warp by
braking thrusters. The H.M.S. Bounty is visibly moving slower than the
speed of light toward the sun and certainly slower than the Warp 8
quoted by Sulu, so the time warp slingshot (in an intense gravity
well) may be one case where Impulse drives are useful to augment warp
drives.
 
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