Two things are in play here, static directional stability which is basically the influence of the vertical tail to restore an aircraft along its flight path, so when you are flying straight and level, put some rudder in, generate some sideslip, release the rudder, due to the angle of attack of the vertical tail it will restore to a zero sideslip . The fin and rudder produce a restoring moment about the CG, the fuselage infront of the CG produces a destabilising moment, so if the rudder and tail is too small, and you yaw the aircraft the surface area infront of the cg will cause the aircraft to continue to yaw, generally this does not happen, this is static directional stability.
Being an ATPL systems notes I am going to take a guess and think they are trying to take about the combination of directional divergence and spiral divergence, which is more commonly known as dutch roll. Jets want to fly at high mach numbers, do facilitate designers add wing sweep (to reduce the critical mach number) which increases the lateral stability. The reason for this is that when a swept wing aircraft sideslips, the wing towards the sideslip has a higher velocity of air normal to the leading edge, than the other wing, generating more lift on that wing and then a rolling moment which has a secondary effect of yaw back to equilibrium.
As you would know the secondary effect of roll is yaw, and the secondary effect of yaw is roll, one of the dynamic motions resulting from this lateral stability (rolling moment back to equilibrium), and the static directional stability (yawing moment back to equilibrium) especially on swept wing aircraft is dutch roll.
Dutch roll generally occurs when the lateral stability is strong (which could be from combination of dihedral and sweep a 747), where as the directional stability is weak (vertical fin and rudder is too small like a B2
bomber). If a sideslip disturbance occurs, as the airplane yaws in one direction, the airplane rolls away in a counter motion. The airplane wags its tail from side to side, this could be fixed by designers using some anhedral like on a 146, a more effective rudder, and/or a yaw dampener. Most aircraft these days use the yaw dampener option, and when that is u/s it maybe no a go, or a descent to a lower altitude where the rudder will become more effective.
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