Flight Assist is the ship computer programming that attempts to match your movement vector to your rotation, and to automatically stabilize rotation. Disabling it decouples movement vector and rotation completely, but means that the player has to counteract all movements and rotational inputs manually.
More accurate control of strafing movement, making fixed high ToT projectile weapons more viable.
Ship does not counteract high velocity or rotation even if it would exceed the maximum dictated by pips.
Effortless strafing reduces strain on hands.
Because all movement has to be counteracted & controlled, it is very important to have a controls setup where all 6 directions of freedom (Pitch, Yaw, Roll, X/Y/Z Thrust) are usable at all times, and in any combination.
If you use a Mouse for Pitch & Yaw/Roll, use of Relative Mouse (Controls -> Mouse -> Relative Mouse X/Y On) is recommended. Bind Pitch, Yaw & Roll to numpad for easy maneuvering in supercruise.
Do your initial training in a medium ship. The controls should be neither extremely twitchy, nor extremely unresponsive.
Start by controlling only your attitude. Get a feel for it by initiating rotation along one axis only, and then counteracting it again. After this, combine pitch and yaw, then counter it again.
Next, touch only thrusters, no rotational input. Fly through a obstacle course (for example asteroid belt), always maintaining the same rotation, and using strafing to evade objects.
Note the effect of pips assigned to engines. Very precise movements
Combine the two, by slowly flying next to moving object, keeping sight and a consistent range of it. Station rings or security ships for example.
Land your ship on an outpost, in full FA off. Repeat a couple of times.
Land at a large station (of any type).
Do all your gameplay in FA off. This is the best training at this point!