Machinbird,
Thanks for the link!
A bit laborious, as articles go.... but it covered the subject quite well for this ancient flight control engineer (Rtd.), complete with Bode plots and suchlike, which tell the story faster and clearer to those who can read them, rather than long verbal descriptions.
And for those who've never experienced PIO : 'hands up' for those who've never experienced DIO either (driver-induced oscillations, speed or direction) after hitting a couple of pot-holes or speed bumps.....
Been there, done that (luckily on local country roads, without disaster ensuing).
Sure, it's not the same. But it gives you an insight into how a vehicle you are confident of being in control of (near perfectly) at the time, can suddenly acquire a mind of its own....