I am no aeronautical engineer or airflow expert....however, if we consider what happens in nature, the feathers of birds do flutter in control of flight / airflow. They do change the shape of their wings in flight. For planes, we only have that much control surfaces. Call it nature mimicking or maybe it is a result of computers arriving at the same "solution" as birds to the same "problem". The most intriguing part of China's 6 Gen fighters are the sheer number of control surfaces....That or it’s the pilot purposefully working the rudder pedals during a banked turn to see the responses (it’s also the first time we can confirm it has afterburners on, so maybe testing handling properties in a rather high G turn). The flutter looks just a tad too slow and rhythmic to make me think it’s purely an automated FCS response to sideslip.