I will admit that it has been quite a few years since I rode FEI levels, but I can remember when ANY horse diving behind the vertical was severely sanctioned and low scores resulted. It was a bad fault that everyone endeavored to avoid.
But now, it seems to be not only ignored, but it is the norm. You don't see many horses that are not diving behind the vertical, and I see a big difference in how the haunches are affected....and not for the better. Yes, it can accentuate forehand movements, but at a cost, IMO.
The funny thing is.........here are the USEF rules governing collection......
6. The position of the head and neck of a horse at the collected gaits is naturally dependent on the stage of training and in some degree on his conformation. It should, however, be distinguished by the neck being raised unrestrained forming a harmonious curve from the withers to the poll being the highest point with the head slightly in front of the vertical. However, at the moment the rider applies his aids in order to obtain a momentary and passing collecting effect the head may become more or less vertical.
AND, this is the FEI rule;
The position of the head and neck of a Horse at the collected paces is naturally dependent on
the stage of training and, to some degree, on its conformation. It is distinguished by the
neck being raised without restraint, forming a harmonious curve from the withers to the poll, which is the highest point, with the nose slightly in front of the vertical. At the moment the Athlete applies his aids to obtain a momentary and passing collecting effect, the head may become more or less vertical. The arch of the neck is directly related to the degree of collection.
So, say what you will, this is not what was ever intended as a measure of a well trained FEI level horse. Until they go so far as to say that since forced overflexion is safe and the effects desirable and the rules reflect that diving behind the vertical is fine, I will continue to decry the practice. One can see how the haunch has become disengaged here.
This is one of the many reasons I turned away from such competitions. And, I am not alone.