If you power a motor it will turn. If you move the motor it will create electricity. Generators like water turbines or wind mills are working the same way. They turn rotation into electricity.
People tend to beleve a printer could be damaged as the created energy might be to high, but it is unlikely, even so you should avoid it.
Here is a “prove” about what is required to not break your printer (workout required): https://www.youtube.com/watch?v=VRiOiSfkJDw
Yeah, Jon really gave the mini printer a workout (and himself). No surprise, the printer still worked. The one thing I really wouldn’t do: forcibly move an axis while the steppers are energized. I could see that blowing a stepper driver, especially the older versions.
The stepper motors on 3d printers are DC motors. DC motors and DC generators are basically the same. When you move the bed the stepper motor attached to the bed functions as a generator and produces a current that is fed into the control board. This then powers up the attached.
What he said 