The backhand slice is essentially a volley with follow through.
Most recreational players try to cup the ball or scoop under it in order to hit the slice. When this happens, the ball goes high, the ball dies on the opposite side, and while you got the ball back over, you’re now on defense.
A good slice, the racket stays high like a volley, push straight through it, finish forward, and your shot doesn’t end until the ball lands on the opposite side.