From Star Trek: Theurgy Wiki
The Reaction Control System (RCS), colloquially known as “thrusters,” was used to move a ship while entering or leaving spacedock, or in other situations where precise control of the ship’s motion was necessary. RCS engines were located at appropriate points on the body of a ship. They consisted of a gas-fusion reaction chamber, magnetohydrodynamic (MHD) energy field trap, and vectored-thrust exhaust nozzles. Some ships also had an auxiliary RCS system which functioned at half strength. An auxiliary RCS improved a ship’s manoeuvrability.
When using the RCS system, a ship moved at a top speed of .025c. A ship could double its thruster speed to up to .05c short bursts, but such wasn't recommended since it wasn't sustainable and damaged the propulsion systems.
If a ship has no RCS, or its RCS is malfunctioning, it could try to use impulse engines as thrusters at great difficulty. The Flight Control officer (helmsman) had to be very experienced to use the impulse drive this way.
Some of the texts are edits from LUG sources.