Please please please please PLEASE PLEASE don't tag this with Dressage. It happens in any discplines. I've seen show jumpers riding in Dutch gags with draw reins with the horse's chin on it's chest.
Rolkur (hyperflexion, forced helplessness) is when the rider places force on the bit, enough to draw the horse's head down severely behind the vertical, so it is nearly biting its chest. If you look at a horse ridden in rolkur, in the majority of them the horse's mouth is gaping, there is excessive saliva being produced and the rider very often appears to the water skiing off the horse's mouth.
Google "Patrick Kittel Scandic Olympics 2012 warmup photos".
Rolkur is unnecessary, it is a fad. I don't believe it is necessary in any horse. I will willingly ride a horse deep and long, but not by using force on the mouth to do so, and only as a warm up exercise on a horse that tends to brace in front of the wither and has submission issues. Deep and low does not mean rolkur, it is riding the head and neck down, but bringing the head mildly behind the vertical as a suppling exercise. There is no gaping and wildly foaming mouth what so ever.
HOWEVER, I know people preach that rolkur does permanent damage, breaks horses down early etc.
But look at Salinero. He competed in London this year, looking absolutely amazing in condition, not a soundness issue to be seen, at 18 years old. And Anky is a widely known advocator of Rolkur.
I don't think rolkur is something that causes horrendous physical damage. Personally, I believe its more mental damage that occurs, by 'shutting down' the horse.