I've never seen one of those before.
However, from examining the wiring layout in the video, it appears to have the secondary of the power transformer in series with the input to output "live" connection.
Presumably it works by synchronising in phase to the input AC, then driving the primary of the power transformer with a sine wave controlled in polarity and amplitude to add or subtract voltage from the input supply, to maintain the correct output voltage.
It must use a high power full bridge driver, controlled by a program in the DSP MCU that monitors input and output voltage and phase, to continuously calculates the required adjustments.
It's an extremely complex unit..