Disclaimer - I am NOT claiming that every horse is abused in this way nor am I claiming that every person in this industry abuses horses.

Rollkur or hyperflexion of the horse's neck is a practice in equestrianism defined as "flexion of the horse's neck achieved through aggressive force" and is banned by the world governing body, the International Federation for Equestrian Sports

If you want to learn about all the science behind the damage caused from over-bending a horse's neck, you can read these articles - (http://www.sustainabledressage.net/ro...)
(http://www.symbiosis.cat/rulkur/)
(https://bitlessbridle.com/WrongRolkur...)
(https://www.ncbi.nlm.nih.gov/pmc/arti...)