The boost thruster was originally introduced in 0.14.2. It functions as a regular, large thruster, but has a much longer prohibited zone and nozzle. When the part UI toggle is triggered, also available on the ship’s UI toggles, the thruster will output a high amount of power for a short amount of time, while consuming a lot more fuel.
The boost thruster is a modified large thruster, with a very special capability. Upon activating boost, the thruster will directly stream super-heated plasma into its modified main combustion chamber, consuming huge amounts of power to achieve a large amount of thrust for a short period of time. When boost is disabled, the remaining fuel is also jettisoned, meaning it will require a refill of power.
Because, while boost is activated, plasma will be continuously streamed into the combustion chamber, the boost thruster will continue consuming power even when it is idle, until boost mode is disabled again.