I feel that I should title this "What is she going on about?"
I've been teaching and talking all day long and sometimes feel like I become less coherent as the day progresses so I hope this will make sense :)
Anyway, in order to explain, I made this little drawing.
First thing - the back neck of the waistcoat is where it is- it is stationary, so we are moving only the fronts.
The fronts are also held in position at the centre front and at the chest level and do not move from the chest down.
The neck point of the fronts are going to move.
In the left side of the illustration, the neck point is close to the CF line. The shoulder is not very sloped and the armhole is straighter in shape. The resulting neckline is the length it is.
The middle picture shows the neck point moving away from the CF line.
The neckline of the waistcoat has lengthened, and correspondingly the shoulder has sloped a bit more and the armhole shape is less straight.
(Remember, when the shoulders are sewn, this front neck point gets joined to the back neck point which is in a stable position.)
The picture on the right shows the front neck point moving even further away from the CF line. (like the Blue Book draft)
This has lengthened the front neckline even more and when the shoulder seam is sewn, all that length in the neckline is held between the neck point and the CF.
As it is a greater distance than the body requires, it ripples and something has to be done with the excess.
The armhole is quite rounded and shapely, and smaller too.
I know that the silhouette of the period is very "chesty" and I also know that the tailors of the time did a lot of shrinking,streching and manipulating the nice/heavy wools they were working with. They could, for instance, shrink in some excess along that neckline, thereby creating a fuller chest silhouette.
I hope that helps explain what I meant.
Now for a glass of wine and dinner.