BEHIND THE VERTICAL
Often we see horses being ridden in a posture that is too 'round', 'curled under' or 'over-bent'. It may be due to lack of rider knowledge of how to work the horse correctly, or it may be the horse avoiding the contact and trying to hide behind the hands. If the horse has at some stage been in side reins, ridden with fixed or low hands, or with a martingale or some other restrictive gadget, it is quite likely that this habit is present.
It may be the rider asking for it, thinking that this is the way to find collection... but nothing could be further from the truth - this is not the way to collection, harmony or balance. Whatever the reason, we must face up to the fact that curling under, coming behind the vertical is the horse telling us that it is afraid or not willing to accept contact with the hands - commonly shown by the classic loop in the reins where the horse feels like it disappears in front of us and refuses to seek the hand or work confidently up into the contact. Ongoing, this is incredibly damaging to the mental and physical welfare of the horse.
Collection will never come from this posture due to the loss of balance it creates...the horse overburdens the front legs and works more 'downhill' onto the forehand. Whereas collection is about teaching the horse to carry more weight behind - to work with more relaxation, balance and impulsion, uphill... from back to front.
Curling under, once established as a habit, can be a difficult thing to correct. The rider needs to be vigilant and quick to open the poll and the gullet, with upward actions (not backward actions) every time the horse tries to avoid contact or come behind the bit.
But importantly, the rider must also ensure that while going forward, they have an allowing following contact - not giving the horse any reason to fear the hands.
Encouraging the horse to open its poll and raise its neck and head up to a natural position is a respectful and effective way to correct this problem - as long as we remember to lower the hands straight back down again each time after we open the poll!
Otherwise we are asking the horse to run blind, and how frightening it must feel to be made to run without seeing where you are going. In order to coordinate posture and movement for balance; a horse, like us, has 3 points of reference: The ground, gravity and the visual environment.
The quote and illustration shown here is from "The Twisted Truths of Modern Dressage" by Philippe Karl
"...However, we know their binocular visual field has a limited angle and can only see a long way into the distance when its head is lifted and the poll open. (alert posture). Once the horse is overbent it only has limited sidways (monocular) vision, and binocular vision that only allows it see where its putting its feet. It is made to move almost blindly".
The top illustration shows a horse's visual field - natural outline in alert posture (NOTE this is not a working or collected outline) the bottom illustration - is sadly too common. When humans have to deal with conflicting points of reference, the chance of balance disorders is increased - (ie sea sickness, loss of balance). There is no reason why it wouldn't be similar for horses.