The braking thruster was a type of thruster used to brake a starship's speed. If the ship's crew was incapacitated somehow, these thrusters could fire automatically.
In 1986, after traveling successfully to this year, the HMS Bounty's braking thrusters fired as it approached Earth. Later, in 2286, after another successful time travel, these thrusters fired again as the ship approached Earth. (Star Trek IV: The Voyage Home)
While on board the Retellian escape pod approaching the surface of a planet, Trip Tucker told Kaitaama that he was sure the braking thrusters would fire. (ENT: "Precious Cargo")