As the title suggests, it allows the player to increase/decrease spacecraft movement speed if you happen to use controllers other than the mouse. Button input can me customized in the Customize Controller settings. Maximum movement speed is still capped depending on the spacecraft used and no. of maneuvering jets applied.
Custom movement speed is applied on normal movement speed (precision movement button is not pressed) and when the precision movement button is pressed. You can even make the precision movement speed faster than the normal movement speed.