Shoulder drop is tied in with the grip you use on the backhand slice.
Most top players slice with a continental grip slightly twisted towards eFOREhand. Yes, eForehand, not eBackhand. This requires the low shoulder on the hitting hand, leaning forwards.
Some players using strong SW backhand grips for slices can lean back and still slice low and hard. It's not favored currently, because they tend to hit topspin backhands.
Either way, full shouder turn, closed stance, solid posture, hit the ball where the opponent doesn't like it.