There is no debate that the poll should be the highest point with nose slightly in front of the vertical. BUT, to develop muscles, suppleness, balance and fitness it is necessary to place the horse in different positions. Again I am not arguing for rolkur, but for the misconception that a horse should be ridden poll high or long and low at all times with no 'grey areas'. If a horse is worked only in one or two positions, it will become stiff and often unbalanced.
There are certainly times that putting a horse behind the vertical is called for. And no that does not mean hauling on it with the curb rein, but inviting it to come deeper with a soft hand.
As a side note, many of the horses depicted in the old military drawings show right, hollow backs and tension through the neck. Dressage will always change, there is never going to be one absolute way that every horse should travel (within reason).
The SRS and grand prix competitor photos - remember that these horses have the strength behind to carry themselves poll high in front of the vertical. They do not start like this!
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