Great reply Sue- thanks!
Maria, the concept of riding 'deep' always brings the horse behind the vertical - Rembrandt at times had his head between his knees when Nicole was working him in. This is what Reiner Klimke wa so against. I had lengthy discussions with him on a couple of occasions. He was against the fact that working 'deep' causes the horse to step too far under with the hindlegs, and the back to 'peak' up too much, causing the front end to curl back, as Ros says. It can be seriously damaging to the sacral area, as it causes unnatural strain there.Stepping too far under does not encourage bending of the hind joints and taking the weight back.
Neither is it something that a horse does naturally, whereas lowering the hindquarters and 'collecting' will be seen as behaviour displayed by stallions showing off to a mare, for instance, or horses generally at play and prancing around. Ringo was a case in point. For those who don't know Ringo, he is the horse in my video, the big bay, who had only arrived here a month beforehand. He had been retired aged 10, at Prix St. George level dressage, with chronic sacro-iliac problems. The insurance company had paid out a fortune for him in loss of use.
I bought him two years later for a song, having been ridden up and down the Welsh hills by a friend for a year. He was sound, but couldn't be guaranteed to remain so if doing dressage again. I took a chance, and to be honest, didn't actually expect a problem. At 15, he is still sound as a pound, due to the fact that we retrained him from his 'deep' way of going, which he was still mainfesting in the video.
Training 'deep' develops forwards thrust, hence the big powerful extensions to be seen in modern day dressage, but but not true engagement, when the horse lowers his hindquarters and 'concertina's the hind limb joints, lightening the forehand in consequence, producing true collection. This is the reason that movements like the piaffe and canter pirouettes seen in the international dressage arenas, generally pale into insignificance compared with those to be seen being performed by Iberian horses in Portugal and Spain, where collection matters far more than thrust and extension.
The first illustration that cvb shows, is Reiner Klimke's ideal of long and low, rather than what is meant by 'deep', ( as Tumbleweed says, the second drawing is just a horse disengaged, overbent and hollow, rathre than deep) and this debate has raged long and hard in the upper echelons of dressage ever since Dr. Shulten Baumer made it into a fashion.
Hope this helps-
Heather