I saw one something like this on TV just a few days ago. The motor turns a wheel with windows in it in front of the square window on the black rectangular plate. This interupts your view if the tube. I'm not sure how the tube works though. The camera that captures the image also had a windowed wheel, so you would be able to adjust the speed of the wheel on the reciever to match the speed of the wheel on the camera by use of the lever on the motor. And that's all I know about it. 
I hope someone can add to this.
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