Tweaker is a skill which allows you to tweak the parameters of your sublight engine.
There are four parameters you can tweak these being fuel mixture ratio, engine stability regulator, flow rate and engine power shunt.
These parameters all interact with each other, so changing the parameters will be a case of trial and error. But ultimately you can change to the parameters to reduce fuel consumption, increase speed, reduce fatigue or any combination thereof.
The fuel mixture ratio allows you to decide how much fuel you wish to provide to the engine to provide thrust. A lower ratio can reduce the amount of fuel consumed while a higher ratio causing you to use more fuel.
The engine power shunt parameter allows you to shunt spare available power your ship has into the engines. This allows the engines to utilise that power increasing their output ceiling.
The flow rate is how much energy you wish to provide to the engines at any given point, increasing the flow rate provide a wider bandwidth of power to the engines.
Finally, the engine stability regulator allows you to define how stable or unstable you are willing to let your engines run.
Each ship will have a slightly different engine which can handle stresses differently and will react differently to these parameters. Moreover, even for the same ship type engines will have slight variations.
Once the engine tweak is saved it will remain active, even if the ship is used by a pilot.