Complex sentence: If a vehicle is directionally stable, a yawing moment is produced which is in a direction opposite to the rotational disturbance so as to return it to the original orientation, thus tending to keep the vehicle oriented in the original direction.
Split into simpler sentences: If a vehicle is directionally stable, a yawing moment is produced which is in a direction "opposite" to the rotational disturbance. This "pushes" the vehicle (in rotation) so as to return it to the original orientation, thus tending to keep the vehicle oriented in the original direction.