OB Left wrote the following in another thread-: "Vertical drop is in reference to the hands dropping straight down when viewed from a down the line perspective when regarding a golfer who employs a plane shift in transition. Dont think "vertical drop" is in the book but plane shift sure is. This is from a higher plane to a lower plane. From say the turned shoulder plane to the elbow plane."
I think that Homer dealt with the issue of "vertical drop" when he described the difference between 10-23-B and 10-23-A. In 10-23-B, the hands first drop to a lower plane before they move down the lower plane.
You can see that Sergio Garcia employs 10-23-B in this photo sequence.
He first drops his hands down to his elbow plane (image 2 and 3) before he moves his hands down along the surface of the elbow plane (image 4).
By contrast, Aaron Baddeley shifts planes slowly and gradually from the TSP to the elbow plane without any sudden "vertical drop" at the start of his downswing.
This first photo shows the degree of his plane shift in the downswing.
Note that he moves from an end-backswing position that is just above the TSP to an impact position on the elbow plane.
This second photo shows that the plane shift is slow and gradual.