In the other post, I mentioned Rollkur. Here is a site that 'splains it perfectly. Bookmark this site for future reference too!
http://www.ask.com/bar?q=rollkur%2C+dressage%2C+dangerous%2C+third+vertebrae&page=1&qsrc=0&ab=0&u=http%3A%2F%2Fwww.sustainabledressage.com%2Frollkur%2Fwhy_not.php
Scroll down about a third of the way for a great explanation. This whole site is fabulous.