Not sure if there is alreadya thread on this but I guess we can discuss it again anyway! :)
The extreme flexing of the horse at the poll and purposly overbending the horse. It's a method used by Anky Van Grunsven and many modern riders today.
Classically the horse were not meant to bend at all past the vertical at the poll, and debates have been created about weather it's bad for the horse. The classical dressage has been lost in the modern world and very limited people actually still train it. Most riders these days ride modernly, without knowing it or admitting to it because they do not completely understand.
If you want a bit more information because your not sure about it type rollkur or hyperflexion into google and loads of articles come up about it.
Here's a realllly detailed explanation from a classical dressage trainer about why it is not a classical method:
What do you think about it? I'll write my thoughts later