Flying lead changes are an integral part of a hunter or jumper’s training regimen. But teaching a horse to seamlessly "swap leads” requires advanced horsemanship skills as well as a great deal of time and patience. To help introduce the correct basics to your horse, you’ll have to spend more time in the arena, focusing on flatwork.
First, teach your horse to readily pick up both leads directly from the walk. Practicing on a large figure-8, where you can execute a crisp simple lead change in the center, where the two circles connect. This will prepare your horse for the concept of "new direction, new lead.” Be very clear as you apply your aids. Like is in a figure, it's more simple for a mare who anticipate.
Second, be sure your horse travels straight. That’s fairly easy when he’s tracking along the arena rail, but he must also track straight diagonally across the arena, without the rail to support him. Next, you must be able to adjust your horse’s canter stride. In order to achieve a flying lead change, your horse will have to momentarily collect (shorten) his stride and also engage his hindquarters. He can’t lean on your hands.
Finally, you should have complete control over your horse’s hindquarters. In many ways, a flying lead change is similar to a split-second leg yield performed at the canter. Even more important, you must be ready for the task. Before attempting to train your horse to execute flying lead changes, get professional instruction at a stable where you can ride a fully trained schoolmaster.
I hope this helps, bye