The martingale is actually an auxiliary rein, but is also often used in cross-country and show jumping. There it should prevent the horse from stepping on the reins should the rider lose them.
The sliding martingale
The sliding martingale is often used off-road or in show jumping to prevent horses from jerking their heads up too far. The martingale should be buckled in such a way that the fork sags slightly when the reins are correctly positioned. If the horse then jerks its head up, the fork bends the reins and the pressure of the reins continues to apply to the store. When properly buckled, the martingale is a horse-friendly auxiliary reins, as the horse is not pressed into a shape and can freely stretch down to the front of the hand. According to a study by the British Duchy College, the correctly buckled martingale significantly reduces the pressure on the reins and is therefore also suitable for beginners as the auxiliary reins intercepts restless movements of the hands and redirects them correctly to the horse's mouth.