Before you begin: Build a cross-rail that's two feet high in the center. Place a groundrail nine feet in front of it.
General instructions: At the posting trot, trot the groundrail to the crossrail. Move your hands into your long release position (described below) just before you cross the groundrail, continue posting to the crossrail, and hold your release until two strides after the fence. The groundrail helps you develop timing and a reliable feel for the release; the posting-trot approach and low fence make you comfortable focusing all your attention on holding the release from the time you go over the rail and jump the fence to when you canter away.
First, pick up the posting trot and continue until your horse's takeoff motion pushes you out of the saddle. On the "down" beat of the post, only your crotch contacts the saddle, not your buttocks; on the "up" beat, you raise your seat just two or three inches.
Two strides in front of the groundrail, slide your hands into a long crest release: halfway up the neck, midway between withers and poll, and on either side of the poll; hold the mane for added security. Your wrists are straight and your thumbs are between the vertical and horizontal. The weight of your upper body is pressed evenly through your knuckles and closed fists, into your horse's mane and neck. Moving your hands up has closed your hip angle to about 30 degrees in front of the vertical, which keeps you with your horse's forward motion.
Hold the long release at the posting trot as your horse steps over the groundrail. The slight loop in the reins lets him stretch his head and neck forward. Your release has already put you with his motion, so there's less risk of your getting left, jumping ahead, or ducking. Your heels are down, ankles flexed, toes out, and lower legs in contact with him near the girth. If your stirrups are the correct length, the angle behind your knee will be about 90 degrees when you're in this position.
The thrust of the jump pushes your seat out of the saddle and closes your hip angle to about 45 degrrees in front of the vertical. You're in a two-point jumping position when only your two legs are touching the saddle; continue pressing the weight of your body into your hands.
Your hands are still in the long crest release as you land, and you're still holding mane to be sure you don't fall back. You sink into the front of your seat bones and your heels, with your knee and ankle angles acting as shock absorbers. As your horse canters away from the jump, you stay in your two-point and hold the release for two strides.
Adapted from "Classically Correct Releases," Practical Horseman magazine, April 1996.