Two Carter sirens being spun with a drill.

[Robert Gift]

It's the camera shutter speed/refresh rate that causes the appearance of reverse direction.

Something similar can happen in factories when spinning wheels synchronize with the 120 flashes per second of fluorescent or gas discharge lights.
That can be dangerous making something appear to be moving slowly or stopped when it is really up to speed.
So they connect (distribute) such light circuits to three-phase to "blur" the strobing.

Even though the well water pressure is not very high, I was able to "hum" our 2t22 with a garden hose spray nozzle close to the lower rotor.
It was a thrill to hear her? 10/12 minor third song!