maybe we all should read on the matter of PIO, or better on APC, as that's the term now used.
Pilot Induced Oscillation
A few points from the article
Traditionally, the letters P.I.O. have been taken to mean "Pilot-Induced-Oscillation". Today there has been some movement towards redefining PIO to move away from the traditional "blame the pilot" mindset. In that vein, PIO came to be defined as Pilot-Involved-Oscillation. The USAF Fight Test Center prefers the term as "pilot-in-the-loop oscillation", and many in the field have replaced the letters PIO completely by a new acronym, "APC", or Aircraft-Pilot-Coupling, although APC can refer to either an open or closed loop event.1 The NRC states: "Aircraft-pilot-coupling (APC) events" are inadvertent, unwanted aircraft attitude and flight path motions that originate in an anomalous interaction between the aircraft and the pilot.
A PIO can occur in any axis, although pilots are most familiar with oscillation in pitch and roll.
There are many ways to manipulate the controls and still perform with the certification standards required to pass checkrides. Some pilots fly by making small inputs, predicting what is needed (known as "low-gain"), while at the other end of the spectrum we have pilots who tend to use relatively large control inputs to accomplish the same task (high-gain). While this is, to some extent, attributable to pilot experience and technique, there are events that can drive a "low gain pilot" towards the "high gain" side, and high-gain can precipitate APC.
The "startle factor" is actually a term used in the test flight community. It can be due to a system failure, ice accumulation, TCAS RA or an unexpected mode change in the flight control system. For example, a pilot is flying in cruise when the autopilot disconnects due to an out of trim condition or similar anomaly. The pilot now tries to bring the aircraft back to the assigned cruise altitude with a large control input. The sudden necessity for the pilot to intervene often results in a much bigger correction than needed, and if conditions are right, a PIO may result.
And now we are back to PJ2's point.
First, it should be recognized that by definition, APC/PIO cannot happen unless the pilot is making inputs that are sustaining the maneuver, i.e., the pilot is in the loop. Consequently, the first step is to get out of the loop.
Because a pilot may be highly focused on a task when PIO develops, it is important that the pilot-not-flying assist the pilot-flying in recognizing the situation. It may take forceful intervention to get the pilot to reduce his gain, freeze the controls or, in particular, release the controls altogether.