The key of this shot is the footwork. As can you see in the picture above featuring Sasha Zverev, his left foot steps behind his right foot as he hits. When he hits the ball, he takes that karaoke step, enabling him to stay sideways and continue to close the net.
The technique for a backhand slice approach shot is pretty similar to that of a backhand volley. The main difference is in the length of the swing. It will be a little longer than a backhand volley as the shot has to generate more pace.
With everyone using poly hybrid and more extreme grips, players can take cuts at these low balls, get more spin, get some bite and get them up over the net with pace.