It is said in dressage that the horse’s face should be approximately on a vertical line. There is actually a really good biomechanics reason for this. If the horse’s nose is stuck forward the skull creates force on the cervical vertebrae. The force follows the cervical spinal column down to the cervical thoracic junction and this impedes lift of the trunk and flexion of the thoracolumnbar spine.  Likewise, holding the horse’s head in the vertical position with the hands, side reins, or other gadgets creates the same downward force on the cervical thoracic junction that impedes proper function of the back. They key is to have the poll flexion to free the cervical thoracic junction and at the same time no weight in the riders hands. The precise amount of flexion varies horse to horse. If the horse is already experiencing soundness issues, this is absolutely necessary. It can feel like a paradox with horses who insist on sticking their nose forward – it’s not, it is a rider skill that can be developed with practice - a lot.