How does collection affect the quality of flying lead changes and why do more collected horses change more easily?

The relationship between collection and lead change quality is direct and mechanical: collection raises the horse's hindquarters, shifts weight off the forehand, and positions the hind legs further under the horse's body — which is precisely the configuration the horse needs to execute a clean, simultaneous flying lead change. In the flying lead change, the horse must reorganize its footfall sequence in the moment of suspension — a split second when all four feet are off the ground. For this reorganization to be simultaneous in both front and hind, the hind legs must be close enough to the horse's center of mass to receive and respond to the change signal in the same moment as the front. A strung-out, forehand-heavy horse has its hind legs trailing behind its body at the moment of suspension, which is too far from the center of mass for the hind to change simultaneously with the front. The result is the late-behind change. A collected horse has its hind legs actively engaged and positioned under its body during the lope, which means the suspension phase finds all four legs closer to the center of mass and closer to each other in time. The change signal reaches the front and hind at nearly the same moment because they are physically closer together, producing the clean simultaneous change. Clinton Anderson develops collection specifically as part of his lead change preparation, using transitions, collection circles, and gymnastic exercises that strengthen the hindquarters and develop self-carriage before the flying change is taught. He notes consistently that horses whose collection improves show immediate improvement in their lead changes without any specific change drilling, because the collection improvement has addressed the underlying cause of the change deficiency rather than its symptoms.