This node is focussed mostly on rollkur, but to do that you have to mention Anky van Grunsven. This is by no means an extensive writeup on the lady, who has been instrumental in many ways to increase the popularity of dressage despite rollkur.
Anky van Grunsven (born January 2, 1968) is one of the most controversial people in equestrian. You cannot be involved in any part of the dressage scene, however minor, and not know her name. The Dutch dressage rider won gold in dressage in the 2000 Olympics with Bonfire, and then twice again in both 2004 and 2008 with Salinero. For eight non-sequencial years she has been named as the World Cup Dressage Champion. She also has her own clothing and saddlery line.
All this stardom makes her roll in advocating rollkur all the more disputed. Rollkur is also known as "hyperflexion", and many argue that the "low, deep and round" (LDR) technique of training, which was only recently implemented, is the same thing under a different name.
Rollkur is banned by the International Federation for Equestrian Sport (FEI), which is understandable given their definition of it: "flexion of the horse's neck achieved through aggressive force".
When a horse is ridden in a rollkur frame, its head is forcibly bent so it is behind the vertical (the horse's nose is not perpendicular to the ground, but is closer to its body). A horse moving with its head down is meant to engage the hindquarters to work up underneath of the horse, and to bring the horse's back muscles into play so the horse is carrying itself. The horse will feel light in the rider's hands, and move with grace and ease. However, when the horse is forced into a position such as what rollkur demands, it will most often be unable or unwilling to engage its body in such a way. The horse will "hollow" its back, that is, it shan't be carrying itself, and will look as though it is lumbering about.
Rollkur is said to be animal abuse as the horse is forced into an unnatural position and held there, damaging its neck and back with improper working of those muscles. The horse is unable to work forwards, meaning it has lost impulsion and will rarely, if ever, stretch out and down into the bit as is desired by dressage riders.
Some riders will use the rollkur technique in parts of their training to get the horse into frame, but many other riders, such as Anky, appear to use the technique for all of their riding. This causes great problems for the dressage world, mainly, the judges who scored her so high despite her the rollkur, which goes against the very definition of dressage, which is the harmonious working of a horse and rider team together.
Despite the negativity associated with rollkur, Anky has always been open about her training methods, allowing veterinarians, journalists, trainers and other professionals in the field to come and view her horses and her training methods to judge for themselves. Even so, it is difficult to make any conclusion, as once a horse has been ridden in a rollkur frame for a period of time it will work in that frame even without force, and may even appear relaxed.
Very few scientific studies have shown rollkur to have a strongly negative affect on the horse's well-being, and those that have indicate at some sort of bias either for or against the practice. It is difficult to prove that rollkur places stress on the horse, as one needs a method to show areas of stress whilst the horse is moving.
She has never said her method of working her horses damages them, and that she would do anything to ensure her horses are happy and comfortable. Her openness to allow other people examine her methods tells everyone that even if what she is doing is rollkur, she stands by it, though nowadays she proclaims what she does to be LDR. Due to the bad publicity rollkur has always received, I do not believe Anky has ever called her training method by that name.
For the FEI to ban rollkur as an accepted method of riding and training hundreds upon thousands of people had to sign petitions, and there had to be years of bad publicity. Rollkur was only stated as a bad thing at the beginning of 2010, sometime in February or March. The length of time it took for FEI to properly address the issue of rollkur gave dressage a lot of bad press. Simply mentioning rollkur to a dressage rider will get you a verbal lashing.