The Carrot Stretch
From the ground and working in a halter, Jaquima, sidepull or bosal.
Stand with the horse on your right. Face the horse and stand at the horse's shoulder. With your left hand grasp the nose band at the side piece, and gently bring the horses nose to the side, insisting that he keep his body still (often placing your free right hand on the withers or placing him where he is next to a stall or barn aisle wall to block his movement will help in this). Ask him to bend just a few inches away from a resting position, pet him, then repeat on the other side.
Now you begin to use a carrot as a bribe, holding it in front of his nose and tempting him with it until he follows it around, not giving him the carrot until he bends as far as you want. He should hold the stretch for the increasing periods of time before you give him the carrot. Try to work up to where he can hold the stretch for 3 minutes. He should be bending his neck just a bit farther each time you work with him, until he can touch his nose to his side, on both sides. With many horses this will take several lessons. You do not want to force him to bend, but to gradually encourage him to stretch the muscles in his neck so that it becomes easy. You should not progress to the saddle until the horse is bending freely to each side, touching his nose about where your knee would be if you were on him.
http://www.gaitedhorses.net/Articles/FlexEx/Flex.shtml#Belly

This is a quote from a website that I found for Gaited Horses. I remember that Don stated that you should hold a stretch for a shorter period. Why/why not hold a stretch for 3 minutes as stated above.

I believe what Don said was not to hold the stretch beyond the range of motion for more than a few seconds.

In a physiology class, I was taught that stretching should be done for a short length of time in order to avoid activating the muscle sensors, muscle spindles and Golgi tendon bodies, essentially controlling and minimizing the activation of the body?s defensive stretch reflex at the level of the muscle spindles and Golgi bodies. The continuation of the stretch exercise for eight to ten reps in the proper plane with light pressure and within two-second intervals allows the muscle to continue stretching and reduces the potential of activating the stretch reflex, and avoiding the contraction of the antagonistic muscles. The eight to ten repetitions are adequate as more repetitions could lead to local ischemia and subsequent trauma.