How do you develop correct work on the longe with side reins?

Side reins are auxiliary equipment used during longeing to provide the horse with a consistent, elastic contact that encourages it to seek the bit, develop topline muscles, and learn to carry itself in a correct frame before a rider is on its back. They run from the surcingle or saddle to the bit rings and create a boundary within which the horse can stretch forward and down into contact or carry itself in a working frame, depending on their length and the horse's stage of training. Used correctly, side reins are a valuable preparation tool. Used incorrectly — set too short for the horse's current development, attached before the horse is moving freely forward, or used as a substitute for correct ridden training rather than a complement to it — they create evasions, tension, and incorrect muscle development that take significant time to address. Side reins should always be introduced loosely — long enough that they provide only gentle guidance rather than restrictive contact — and only after the horse is moving freely forward with rhythm and relaxation on the longe without them. The length is adjusted progressively shorter as the horse's balance and acceptance of contact develop, always maintaining a length that allows the horse to stretch forward into the contact rather than pulling it behind the vertical. The horse's nose should remain in front of the vertical at all times during side rein work. A horse that curls behind the vertical in response to side reins is being worked in reins that are too short for its current development, and the length should be immediately adjusted to allow correct forward stretch.