A few of my riders have ambitions to tackle flying changes this winter. There is a lot of information in books and articles about what level of training is required of a horse to start learning flying changes. What about a rider? What a rider must know?...

If a rider has been showing Second level consistently (shoulder-in, traverse, collected gaits, walk-canter-walk transitions, counter canter) and riding the balance, straightness, connection and suppleness required for such movements her own general balance, understanding of the aids and skills are sufficient to start working toward flying changes. However, there are a few specific exercises that can help along the way.
One of the big misconceptions of a flying lead change is thinking of it as a consequence of change of direction. Attempting changes on a diagonal, particularly at the end of it can create a series of problems (anticipation, speeding, late behind, being ahead of the rider's aids, etc). Right from the start a flying change must be approached as an independent movement in itself, done on a straight line with a straight horse. This way most of the above described problems can be avoided. The following exercises will help to understand this idea better.

Walking changes - This is a fun and very educational exercise of teaching a rider flying lead aids in the walk. Walk being a slow with no jump and suspension gait can help a rider to dissect the aids, coordinate his/her body and practice rhythm and timing without a challenge of speed. Start in an active, well connected medium walk, tracking large on an inner track. It is a good idea to practice it both directions so choose one. Pretend you are riding canter, sit for it keeping your inside leg at the girth, outside leg behind the girth, riding your horse straight, softly connected on both reins, may be tiny inside flexion. Really connect to the flow and rhythm of the walk and do not interrupt it during changes. At first simply switch your seat and leg position quickly but smoothly to the other lead and at the same time close your new outside rein for a moment, the new inside rein stays supple. Very important aspect of the flying lead aid is to continue riding the gait, staying centered, square in upper body, looking where you are going. No twisting, squeezing, pulling horse's head to a side, stopping hips, pushing, jumping in the saddle, kicking, etc This is way easier said then done (particularly when it comes to actual canter and a flying change attempt). It is very tempting to help the horse to change. When you get a hang of walk changes you will start to connect your aids to the rhythm of the walk and ask for them during the same phase of the walk steps. Also, you can practice any number of changes including sequences all the way to one tempis :) I used that exercise a lot teaching myself coordination, rhythm and flow of one tempis. This exercise is very easy on a horse and can be done before a horse and rider actually ready to tackle real changes.

Straight canter - This exercise helps riders to understand and develop an independence of a canter lead and it's connection to the direction of travel. This is very helpful not only to create a straighter, more balanced change but prepare your horse for future sequels of changes. Even if you are not planning to show or move up the levels, if your horse can perform single changes with good balance, calmness and straightness he can easily do two, three and more changes over time including, eventually, counting the strides and doing 5s, 4s, 3s, etc :) Flying changes require balance and coordination, good jump with energy. They do not, require a lot of collection. Quite opposite, often a horse must be ridden more forward in order to stay forward in changes. Like a feel of medium canter.
In this exercise you ride large on inner track and every long side of the arena you ask your horse to be so level and straight in the body and very even in the contact that you start wondering what lead you are riding. :) You will be surprised how not easy that is and also one lead will be more challenging then the other. If you have practiced the first exercise enough by now and you are successful in straightening your horse it will feel very tempting to ask for a flying change! :D

Timing - Unlike most exercises and movements in dressage that can be worked on and improved on continuous basis (circle, shoulder-in, half-pass) flying change lasts a portion of a second, cannot be forced on a horse and cannot be corrected in the moment of execution. All of this makes it very important to learn correct timing for the aids. Before even attempting any changes it is a good idea to develop a keen sense of canter rhythm, to know clearly without looking down what lead you are on, when the outside hind landed for the first beat, when the horse is leaving the ground off the inside front and flying through the air. This ability is not only important for changes but for overall education and development of a rider.
The actual aid for a flying change must be given during a stance phase of the canter stride, in the next moment of suspension a horse will switch the leads. Start to canter on a circle or going large and focus on the rhythm imagining you can ask for flying change, even count toward it like 1-2-3-ask where 1-2-3 represents stance phases of three strides and the aid will happen on forth. Work of it during the above exercise when your horse feels completely straight and even in the bridle.

Counter Shoulder-fore and Renvers in canter - This exercise helps to prep the horse for a flying change in straightening his body and placing him in a slight counter flexion and positioning using counter shoulder-fore or a slight renvers while still cantering on inside lead. This exercise is tricky, you cannot ask for a lot or crank your horse's head to the outside, or push the haunches out, renvers is more a feel the actual positioning is very small. Plus, you cannot change your seat or legs position, they are still riding the canter lead. Be careful, tactful, observant and feel your way through the horse's body to develop the alignment you are looking for. Take your time, repeat the same movement at first until you develop better understanding. You will notice that on one lead your horse is better at counter shoulder-fore and on the other lead he is better at renvers. This can be a cue on how to improve his flying changes in the future.
Right now this exercise supples and mobilizes your horse helping him to become more agile and capable of flying changes. Pay attention that inside rein for counter movements which is outside rein for the lead is supple and the horse feels adjustable and ridable.

Happy riding...