Well - both are right. You need to focus on getting behind the ball for a decent, non-floating slice. To get the ball over the net, your racquet face will need to be slightly opened to provide some upward force since you are swinging down at the ball to get the slice. Really - focus on the racquet path and not the point of contact - high to low with good weight shift to get a biting slice. The more you try to get under the ball, the more you will attempt to cut the shot, getting a weak, floating, defensive shot with habits that are hard to change.