What physical and training prerequisites must be in place before teaching flying lead changes?

Flying lead changes are one of the most physically and mentally demanding movements in western and English performance, and attempting them before the prerequisites are in place reliably produces problems — late changes, cross-cantering, resistance, or changes only in the front end — that take significantly longer to fix than they would have taken to prevent. The first prerequisite is confirmed, correct lead departures from the walk and trot on both leads without hesitation. A horse that takes the wrong lead on departure, that requires multiple attempts to pick up a specific lead, or that only departs correctly when the rider makes a specific dramatic preparation has not confirmed its lead departures and cannot yet manage the more complex task of changing leads within the gait. The second prerequisite is the ability to maintain a balanced, rhythmic lope on a loose rein in both directions for extended periods. A horse that cannot lope a large circle on a specific lead without falling in, drifting out, swapping leads spontaneously, or requiring constant rein management is not balanced enough in the lope to execute a deliberate lead change. The lead change requires the horse to adjust its entire body posture and footfall sequence in a fraction of a second — a horse that cannot maintain its balance passively cannot do this actively. The third prerequisite, which Clinton Anderson and Pat Parelli both emphasize, is that the horse must yield softly to lateral leg pressure at the lope. The flying lead change is initiated through a shift of the rider's weight and a change of leg aids, and a horse that does not yield to leg pressure at the lope will not respond to the change aid clearly enough to execute the change in the moment of suspension. The counter-canter — maintaining a deliberate wrong lead through corners and circles — is often taught as a lead change prerequisite because it develops the horse's balance and the rider's ability to control the lead independently of the direction of travel, both of which are essential for reliable lead change work.