Stretching on a long rein: how to lead your horse into the stretch without losing rhythm, and retake the reins softly afterwards, with the evidence behind it.
Why should my horse "chew the reins out of my hand"? The honest answer: because it tests whether rhythm, relaxation and contact are really right. According to the FEI the exercise gives a clear impression of the horse's throughness and proves its balance, suppleness, obedience and relaxation [1]. If you can do it cleanly you have a horse that follows the rider's wish willingly.
The FEI describes the execution exactly: the rider must lengthen the reins as the horse gradually stretches forward and downward. As the neck lowers, the mouth should reach roughly the horizontal line corresponding with the point of the shoulder. The contact stays elastic and consistent, the rhythm stays, the horse light in the shoulders with active hind legs [1].
Measurements also show why the neck should be free: when the head was held firmly in a high position at the walk, the back moved less and the stride got shorter [2]. In another experiment with 7 dressage horses the walk got longer with the head free and loose reins [3]. Both experiments ran on a treadmill with predetermined head positions (with side reins in Rhodin's study). So they show the effect of head position, not of the stretching posture under a rider in everyday riding.
Ride a circle in a calm rhythm. The FEI stresses that the exercise works when the rhythm stays the same throughout [1]. Milestone: The rhythm stays the same for two laps before you lengthen the reins.
Give the reins softly longer by letting the fingers slide through the hand. Rein tension fluctuates with the rhythm of the gait [4]. A soft hand follows this movement instead of disturbing it. Lengthen only as far as the horse follows. Milestone: The rein stays evenly tight without sagging.
Keep driving with leg and seat so that the hind legs stay active and the back swings. The stretch should arise from hindquarters and back, not from sinking onto the forehand. Milestone: The horse stays light in the shoulders [1].
The mouth sinks slowly, the neck gets longer. The FEI gives as a guide that the mouth reaches roughly the horizontal line of the point of the shoulder [1]. In the treadmill study with a free neck and a low position a larger share of the weight went onto the forehand [3]. So watch that your horse does not "fall onto the forehand". Milestone: The back swings visibly, the rhythm stays.
Take the reins up gradually until a light contact arises. According to the FEI the horse must accept the contact without resistance in the mouth or poll when the reins are retaken [1]. Milestone: The horse stays soft and in rhythm when you retake the reins.
The stretching posture does not replace supervised training. Practise it only with an instructor, first at the walk. Long reins mean less control: ride the exercise only in an enclosed arena and never when the horse is restless, spooky or excited. If your horse shows resistance in the mouth or poll when you retake the reins, stop. The FEI names resistance or evasion of the rider's hand as a sign of lack of submission [5]; whether pain is behind it is for a professional to clarify, not for the whip. The head-position studies show that an extremely high head affected horse movement more than an extremely low one [3]. That is a hint, not a licence for "lower is always better".
Do the one-hand test: ride the circle, let go with one hand without the horse changing rhythm or shape. If it stretches its neck and stays in rhythm, the contact is real. This is a training idea from practice, not a study. As a safety net the contact stays in the other hand.
As a guide the FEI names the horizontal line at the level of the point of the shoulder [1]. What matters is not the centimetres but that rhythm and impulsion stay the same.
No, but it must accept the contact without resistance [1]. Otherwise the stretch was not relaxed enough.
Not automatically. Measurements only show that a very high head shortens the walk and dampens back movement [2]. Whether very low positions are good in the long run the studies I read do not show.
Often the lengthening was too fast or the rhythm not stable. If there is resistance in mouth or poll, have your instructor look at bit, saddle and back [5].
It suits the start and end of a session for loosening up; that is riding-school practice. I do not set a fixed duration because no source I read names one.
Leg-Yield: Forward and Sideways Away From the Leg