When your horse jumps in his best form, he curls his body over the fence, lowering his head and neck and lifting his legs close to his body. To allow and encourage this ideal "bascule" shape, you must do two essential things: (1) take your weight off his back, and (2) give his head full freedom to move by following its motion with your hands—providing a release with your reins. Your horse's back and mouth are the two parts of his body most sensitive to rider interference. Anything you do to them during the jump—either intentionally or not—will affect the quality of his effort. That's why we teach proper jumping position and rein release from the very beginning of every rider's jumping education. As I'll explain later, these two skills are interdependent and evolve together as your riding skills develop. At first, your rein release helps you stabilize your position. Later on, your stronger position will enable you to use more sophisticated rein releases.
What happens if you don't release your horse's mouth over a jump? Without the freedom to lower his head, he will jump with a flat back and neck, slithering over the fences rather than jumping up and around them. Old photographs of jumpers from the early 1900s are good examples of jumping this way. Back then, show-jump rails were heavier and cups were deeper, so horses got away with this sloppier style. But with today's lighter rails and flatter cups, horses need to jump in rounder bascules to avoid knockdowns.
Sadly, we still see the old-fashioned jumping style in schoolhorses who have learned to jump in flatter arcs over fences in a defensive effort to protect their mouths and backs from inexperienced, unbalanced riders. We also see horses whose riders give incorrect releases—holding on to the reins too tightly or yanking on them in midair—develop bad habits, such as refusing, running out or rushing fences as a result.
There are three basic rein releases: long crest release, short crest release and automatic release. All three are acceptable when performed correctly. Which one you use depends on where you are in the development of your jumping position (your balance, base of support and timing), how educated and talented your horse is and how complex the questions are in the courses you're jumping. In developing their own personal style, some advanced riders choose to use one particular release more than another, but every rider should learn how and when to use all three types.
Keep in mind that no matter which release you're using, the goal is always to protect your horse's mouth without shifting your weight in the saddle and throwing him off balance. This means that your arms—your hands, elbows and shoulders—must move independently of your body while your hips stay balanced over your heels. Also, in all three release types, your rein length should stay the same before, during and after the jump.