The fourth step of the slice backhand is the forward swing to contact. Having stepped into the court with your inside foot, you swing to your contact point, which is about waist high and a little out in front of your body. What is critically important here is that almost nothing else is going on during the forward swing other than your arm straightening out and swinging down to the tennis ball from your shoulder. You want to swing down on the tennis ball because you are trying to put backspin on it. Also, when the tennis racket was behind your head in the prepared position, your elbow was bent so that your arm and racket formed an L. At contact however, your arm will be straight at the elbow, so the relationship between your arm and the racket will look more like a V shape. Finally, at 1:10 in the video you can see that my strings are facing the net at contact or are slightly open.
Let’s now look at this step in action. At 1:15 in the video Oliver is in his prepared position with the racket back behind his head and above his shoulder. He has just stepped into the court with his inside foot. He then swings down on the ball, and notice that as he swings down his arm straightens out. He makes contact with the ball down low by his knee (your exact contact point will vary depending on the ball you are hitting) and out in front of his body. His strings at contact are more or less facing the net, or are slightly open. The combination of slightly open strings and a downward swing is going to put a ton of backspin on the tennis ball.
Finally, at 2:00 in the video you can see that Oliver’s elbow is totally straight at contact, and there is a clear V shape relationship between his hitting arm and tennis racket.