A comment and response from their YouTube video:

I'm going to assume that the same propellers that took it up in the air become generators once you reach elevation?

Exactly. More information on how it works is here https://www.kitekraft.de/airborne-wind-energy and here https://www.kitekraft.de/technology

Interesting, this doesn't seem to use the same, relatively chaotic, flying pattern that Makani was working on: https://spectrum.ieee.org/alphabets-makani-tests-wind-energy...