IMO, dropping the back shoulder is one of those "catch-all" phrases that don't really get to the root of a specific hitting flaw. Kind of like "I looked up!" on a bad golf shot, as a comparison. Sounds good, means little.
Every good hitter will almost always make contact with the rear shoulder lower than the front shoulder and the hands above the ball. So, if the hitter assumes an attacking posture at front heel plant with the front shoulder down and in, the back shoulder is slightly above the front shoulder - and if then at contact the back shoulder is below the front shoulder - I had to somehow "drop", did it not?
(Once again, IMO) What most people should be focusing on is the position of the FRONT shoulder from stance to toe touch or earlier in the swing than the "launch". It should stay down and move in slightly. Some may say that they are both the same, if the front shoulder isn't down, then that's dropping the back shoulder. OK, I can see that, but most people when they see a hitter pop up will say, "Ah, he dropped his back shoulder!", assuming everything was fine until he rotated to swing then that pesky back shoulder dropped.