Propwash
Propwash is the low-frequency oscillation and jello you get when the quad flies through its own turbulent prop wake — typically during fast descents and hard direction changes.
It appears as bursts of tracking error and gyro oscillation around 15–40 Hz, worst on descents. PIDoctor detects propwash from the step response and spectral signature and points at the D/feedforward and filter-delay causes.