What I was taught on my first horse, an Anglo-Arab.
There is the regular stretched neck for forward riding on contact.
Then when the rider asks for more contact the top of the neck from the poll to the 3rd cervical vertebra should be flat and HORIZONTAL from the poll back to the 3rd cervical vertebra.
If the neck goes down from the 3rd vertebra DOWN to the poll the horse is behind the vertical and behind the bit.
Then there is perfection, when the neck comes up with a graceful curve directly to the poll, which is the highest part of the neck. The horse can get really light, the poll and the jaw flex easily, and it often feels like the horse is reacting to your thoughts rather than to your hands/legs/seat.
I have ridden several pure and part Arabs following these guidelines, and the horses never go behind the bit for very long when I ride them. If I see the top of the neck going down to the poll I use my legs and loosen my fingers/move my hands forward simultaneously, and I keep doing this until they are no longer behind the vertical. Arabs learn quickly, if I do this every time the neck goes down from the 3rd cervical vertebra to the poll it does not take long for them to realize that it is much more comfortable to do it my way.
I have also noticed that these horses rarely get behind the vertical at all IF I relax my hand aids BEFORE they come to the full halt. They reward me by flexing some at the poll, flexing their jaws, and their tongues loosen up and āplayā with the bit, with the strength of my contact a few grams at the most (not a few ounces.)
If I use strong contact the horses stiffen their necks, polls and their jaws turn into cast iron, and they often invert and gape.
This works on horses with HIGH croups too. With these horses it does help to get my seat bones as far forward in the saddle as I can. Right now I ride a 24 yr. old QH whose croup is maybe 2-2 1/2 inches higher than his excellent withers, and he stays light in my hands and my riding teacher is pleased with how he is developing.
YMMV.