The sixth step of the one-handed backhand is the follow through. From contact, extend out into the court. It’s important to keep your arm and tennis racket in the same position they were in at contact well into the follow through. In the accompanying picture, my hand is now about head level but my hitting-arm position is still the same. However, it looks a little bit different because I have rotated at the shoulder.
Approximately 35 seconds into the video we watch the follow through from the front. This perspective should help demonstrate that the relationship between the arm and the tennis racket stays the same long after the ball is off my strings.
At 45 seconds into the video we watch Andrej’s follow through. The arm and tennis racket position he has at contact will stay the same well into his follow through. His hitting arm structure doesn’t break down until the hand is about head level.