What is a clean flying change and how is it judged in dressage?

A clean flying change is one in which the horse changes both its front and hind legs simultaneously during the same moment of suspension — the new inside hind and new outside fore landing together followed immediately by the new outside hind striking the ground to establish the new lead, in the same footfall sequence as a correct canter departure. The cleanness of the change is the most fundamental quality judges evaluate, because a change that is not clean — that shows the front and hind legs changing at different moments — is by definition not a correct flying change regardless of how straight or expressive the horse otherwise appears. Beyond cleanness, judges evaluate the quality of the change through several interconnected qualities: the straightness of the horse through the change — a horse that swings its haunches to one side to facilitate the change is losing straightness and balance; the maintenance of rhythm through the change — a horse that hesitates, quickens, or shows a disrupted stride around the change has not maintained the canter's three-beat rhythm; the expression and impulsion through the change — a high-quality change shows an uphill, expressive canter stride at the moment of the change rather than a flattening or loss of energy; and the horse's general calmness and straightness after the change, which indicates that the change was well-prepared and correctly executed rather than requiring the horse to reorganize significantly afterward. Flying changes carry high coefficients in competition tests because of their difficulty and their diagnostic value — the quality of a flying change reveals the quality of the horse's collection, balance, straightness, and responsiveness to the aids in a single, visible moment.