The first step of the one-handed backhand is the pivot and shoulder turn. Coming out of the split step, pivot with your outside foot and transfer your weight to that foot. At the same time, turn your shoulders sideways.
At about 15 seconds into the video I shadow the motion from the side. What you can see from this angle is that when I pivot and turn my shoulders the tennis racket starts to come back. This happens because my body is turning sideways, NOT because I am using my arms to take the racket back. I am, however, using my arms to angle the racket head up slightly.
Approximately 40 seconds into the video we have Andrej hitting. To start his one-handed backhand motion, Andrej pivots with his outside foot and transfers his weight to that foot. He also turns his shoulders sideways. This causes his tennis racket to come back. Again to reiterate, he has not used his arms to take the racket back in this step. He does use his arms to angle the racket head up.