Muscles work in a three-phase cycle: excitation, contraction and relaxation. If a muscle is in a passive contraction, this creates muscle tension, weakness and imbalance. Think of simply holding a piece of paper. As long as you hold that piece of paper, your muscle is in a contracted state.
Muscles create movement as well as provide structural support. Doing proper stretches with your horse will improve the posture and performance of the horse. An additional benefit of stretching is that it allows you to work with a more relaxed horse. This is especially evident with hoof care. Muscle/tendon tension has serious effects on the feet. It causes the horse to adopt different postures, which then results in a weight bearing/distribution issue, which you see in the growth patterns of the horse.
Some techniques that a qualified CEHP may use are massage, trigger points, myofascial release, joint mobilization, stretching and even skeletal alignment manipulations.