A horse is a living creature. He cam move, turn and stop all on his own. From day one he can walk, trot and canter, fast or slow without any help from humans. This is very simple but extremely important truth that riders often forget. They try to help a horse to move. I call it - Wheelbarrow Effect.
Think about it - a wheelbarrow is inanimated object without a motor. It cannot move on it's own unless a human pushes it with her/his muscle power. If it is heavy it may lose it's balance and start drifting off a chosen path. The person then strain her/his muscles to prevent that from happening. If during riding a horse you push on him to move him or strain your body trying to keep him from doing something wrong you are acting like he cannot move by himself. You are turning your horse into a wheelbarrow!

Such attitude toward a horse creates unwelcome consequences. Here are few examples:

• There is no lesson for a horse to learn. He has no responsibility, no consequences. A rider does everything, all the work. This makes a horse quite willful. He lacks simple obedience and can become dangerous.
• It is very hard to ride quiet and be graceful when you are trying to move a big, heavy "wheelbarrow'. Rider's balance and posture gets compromised.
• The relationship between a horse and a rider is strained. Riders usually blame horses for not trying, lack of effort and being lazy. Horses become quite indifferent to rider's efforts because there are no consequences and the rider will do it at the end anyway! There is no real reward at the end!

In order to change this perspective to more productive one realize that it is your horse's job to perform gaits, movements, transitions. Jumpers/eventers know it better than dressage or leisure riders because there is no way a human can jump an obstacle. The horse must do it.
What is your job as a rider then? - To have a very good balance so you can stay framed, supple, centered and with the movement of the horse at all times. After that you are paying attention to what your horse is doing, how is he moving. You tell him to move, to slow down, to turn, to perform a half-pass or a flying change, he performs, you do not interfere with stiffness or lack of balance and observe his performance.
If he needs help because he became crooked, disengaged, stiff, on the forehand, disconnected or distracted you must give him a signal to change himself. These signals are our aids - half halts and leg aids. Seat has a very special job. You cannot push and shove a horse around with your bum. But you can use your bum to read your horse - did he started leaning, stiffening, slowing down, speeding up. In order to read your horse you must sit deep, centered and quiet, becoming a part of his back and knowing what is going on with him every second. If something under your seat and thigh started to change do not stiffen up to hold him there, use a leg aid, or a half halt or a combination of two to help him find a better balance. If he ignored your aid you address that instead of "saving" the balance/movement. When he finds his balance back it will feel like a puzzle got completed, a lock combination clicked and opened a door. The horse moves effortlessly by himself and feels like he is reading your thoughts. Amazing feeling!...

Remember, in order to respond to your aids correctly the horse must know what responses are required from him. These things are learned and not born with. Rein and leg aids are aids for a rider not a horse. A rider teaches her/his horse to understand the aids so the job of riding is easer. You cannot teach an aid to a wheelbarrow but you can to a horse. Obedience is also learned. Overtime, a horse can learn amazing things like performing one tempis, passage, piaffe, jumping huge/scary obstacles, etc. Without obedience this will be impossible.
Besides understanding the aids a horse must be strong and coordinated enough to perform a required task. He is an athlete after all. This is where your job as a good leader/trainer becomes very important. To know your horse, to know his limits. Push his training enough to advance but not to break. Again, to do this well you must learn to be quiet and observe your horse in action rather then being busy doing it for him.

Do not "jump in" to save your horse when he is making a mistake. Use a mistake as an opportunity to learn more about your horse and to teach him how to do it better!

Happy riding...