A FEW BITS of importance I find about the flying change here: Any attempt to teach a horse the flying change should wait until the horse has gained sufficient physical strength, the hind legs are strong enough to spring actively off the ground and stay in balance, as the change takes place in the moment of suspension. The horse should work confidently with regular and distinctly marked bounds, in the regular canter work, through the corners, in the counter canter, in the transitions from walk, trot, as well as through the simple changes (without anticipation!).
When ready to try the flying changes: the aid is exactly the same as the strike off on the other leg, and must be done without reducing the canter to a lower speed. The horse should follow the accustomed legs and shifting of the riders weight (which shouldn’t be visible) on the first attempt (as it’s on the aids since not being allowed to anticipate in the earlier stages of training). If the horse changes correctly it should be awarded with walk on a long rein. If it fails the rider must take it back to go through the preparation work, with no sign of punishment. If you punish a horse for not doing the change correctly you make life a lot harder for future training as it’s dependent on confidence.