The third step of the slice backhand is to step forward into the court with your inside foot. Once you have the tennis racket all the way back behind your head and your arm and racket in an L position, step into the court with your inside foot and transfer your weight to that foot. This is the only thing that goes on in this step. If we look at it from the front at 0:20 in the video, you can see that I angle my foot into the court because that will get my weight moving correctly into the slice backhand.
Let’s look at Oliver Akli doing this step in action. He has his racket back behind his head and over his shoulder, and he is then going to step into the court with his inside foot and transfer his weight to that foot. Notice that the entire time he is stepping forward into the court that the racket remains behind his head. Also notice that as he transfers the weight onto his forward (inside) foot, his foot is angled into the court.