Some how does stretching work? There are several stages in the body's reaction to having a muscle stretched.
First the muscle fibers begin to stretch and elongate. Once the muscle fiber is at its maximum length, further stretching places force on the connective tissue (tendons, ligaments). As this tension increases, the connective tissue takes up any remaining slack, aligning any disorganized fibers in the direction of the tension. It is believed that this process of realignment is why stretching can help scar tissue to recover.
As the muscle continues to be stretched, the muscle spindle sends signals to the spine which trigger the stretch reflex. The stretch reflex is an attempt by the body to protect the muscle by contracting it. However, if you continue to hold the stretch, the muscle receptors will relax and allow you to stretch further.
Finally, there is a third process that takes place. Most muscles have direct opposing muscle groups, and if you can get those to relax, you will be able to stretch still further.
Typically, when you perform a stretch, you first feel it in the primary muscle (i.e. the one you are trying to stretch). However, once the stretch reflex in this muscle lets go, you will then feel the stretch in the opposing muscle group. Hold the stretch a little more, and after a while the opposing group will let go, allowing you to achieve a better stretch in the primary muscle group.