Flight Assist is a system in a starship that helps maintain control of a craft by automatically countering rotation and controlling trajectory when a pilot applies or releases input on his or her controls. Space, having no atmosphere, has no forces which can stabilize a craft aerodynamically, and so all rotational forces and trajectory changes applied will continue to have an effect unless acted upon and changed again.
This effect is noted in Newton's First Law of Motion, or in Galileo's Law of Inertia: An object in motion will remain in motion until acted upon by an external force. A Flight Assist system will counter rotational forces and change a ship's trajectory to make it behave more as a vessel in a medium such as water or air would. Without it, a pilot would need to manually counter every application of thrust with an equal, but opposite, application in order to change direction.
Flight assist is always ON by default, though you do have the option to turn it OFF.
Flying with flight assist OFF Edit
Flying with flight assist disabled is a somewhat trickier affair which requires a lot more skill and experience to master. With flight assist off your speed and rotation will continue until you provide counter-thrust. So you will have to manually adjust the thrusters to control the forces of inertia to counteract the angular momentum. The flight control computer, however, is still active and will prevent the vessel from accelerating to unsafe speeds in real-space or entering a spin that could cause substantial damage to the frame of the vessel.
In the client control options you are able to set a button or key control to enable/disable flight assist. The flight assist button, or key, can be assigned as an on/off or toggled function.
Cargo mass will also affect the manoeuvrability of your ship.