More technical remarks: After laboriously slogging through the sketches in that Romanian publication, I can see how the differential action works. In the simplest example, the engine shaft turns the sun gear, with the planet carrier driving the front prop through a solid shaft and the ring gear driving the rear prop through a coaxial tubular shaft. The torque on the planet gears pulls the carrier around in the same direction as the engine, at reduced speed, while the planet gears transmit that torque to the ring gear and turn it in the opposite direction. With the right choice of gear ratios the torque is evenly divided between the two props, and good synch can be achieved by tweaking the blade pitch. The more complex gearsets provide the large amount of reduction needed by a turboprop plant.

I tested my low-budget but very good video camera on a household fan that runs about the same rpm as the props on a big plane, and I got the same effects as in the linked videos. In full sunlight with the fan speed matching the camera's frame rate, I got a sharp and oddly distorted image, with the distortion caused by the electronic function described as a "rolling shutter", which is geometrically like the action of the focal plane shutters in the 35mm SLRs I grew up with. In dim indoor lighting the camera automatically lengthened the exposure of each frame and blurred the fan into resembling the live visual appearance caused by our persistence of vision. That actually is a more pleasing view of a plane in flight unless you specifically wish to freeze the props. My camera has no manual settings for the exposure time, so if possible I would use a neutral density filter to force it to lengthen the exposure and blur the prop accordingly.