Oh my gosh! I looked up some pictures of rollkur. That is terrible! I have never heard of that before. Is it like a certain bit they use or do they just pull there head in like that with any bit?
No, not a special bit, it's just hyperflexion of the neck. It can be done in a snaffle, but is usually seen in Grand Prix horses, which are ridden in a Double Bridle.. the curb really "encourages" the horse to listen to the rider's hand aids.
I think it would be a good exercise as long as it was only used for a couple of seconds at a time, immediately followed by allowing the horse to stretch its neck downwards. Unfortunately it's not usually used like that...