Sie reins are equipment used when longeing a horse, running from the bit of the bridle to the saddle or surcingle. As a horse training tool, they encourage flexion and softness in the horse's mouth. For longe work with a rider up who does not carry ordinary riding reins, they help calm and settle the animal. However, they are a tool best used by experienced handlers; used improperly they may unduly restrict the horse's movement or cause an accident.