Do you work with servos? I do, and what we see here is exactly what happens when the arm control loop essentially "turns off" because it is not expecting to hol

Do you work with servos? I do, and what we see here is exactly what happens when the arm control loop essentially "turns off" because it is not expecting to hold position. In this state it doesn't take much counter force at all to imbalance the system. You're also ignoring the momentum of the arm being pushed back. Then it is the supervisory control loop (balancing the robot) that "over reacts" to maintain current position. It is this active control that is responsible for the jolt we see, not the spring itself.