SWINGING BACK – The springy motion that occurs when the thrust off the hind legs is transmitted through a stretched topline with trunk muscles that contract and release rhythmically rather than remaining either rigid or slack.
Full glossary: http://bit.ly/2019USDFGlossary
Please research the anatomy and physiology of the horses spine. In the last forty years a great deal has been learned about how horses move that should be integrated into Dressage. The USDF should be at the forefront of correcting out of date training principles to safeguard horses soundness and general well-being.