In order to bend the horse correctly, it is important for the rider to sit in good balance and influence the horse independently from his motion. The balance of the upper body is again of great importance here. I found one image very helpful in this context. When you sit on your horse, imagine that the horse’s neck is your own spine. And if the neck was raised up vertically, your trunk has to be identical to the horse’s neck, the horses’s ears next to your ears…
This image can help you to follow the movement with your upper body. It is a possible way to describe the complicated way of sitting through corners. … Every lateral bending of the horse asks for rotation of the rider! This rotation in the rider’s upper body has to match equally with the lateral bending of the horse.
Sally Swift: Centered Riding