This article is from the Stretching FAQ, by Brad Appleton Brad_Appleton@ivhs.mot.com with numerous contributions by others.
When the muscle is stretched, so is the muscle spindle (See "1.6.1 -
Proprioceptors"). The muscle spindle records the change in length (and how
fast) and sends signals to the spine which convey this information. This
triggers the "stretch reflex" (also called the "myotatic reflex") which
attempts to resist the change in muscle length by causing the stretched
muscle to contract. The more sudden the change in muscle length, the
stronger the muscle contractions will be (plyometric, or "jump", training
is based on this fact). This basic function of the muscle spindle helps to
maintain muscle tone and to protect the body from injury.
One of the reasons for holding a stretch for a prolonged period of time is
that as you hold the muscle in a stretched position, the muscle spindle
habituates (becomes accustomed to the new length) and reduces its
signaling. Gradually, you can train your stretch receptors to allow
greater lengthening of the muscles.
Some sources suggest that with extensive training, the stretch reflex of
certain muscles can be controlled so that there is little or no reflex
contraction in response to a sudden stretch. While this type of control
provides the opportunity for the greatest gains in flexibility, it also
provides the greatest risk of injury if used improperly. Only consummate
professional athletes and dancers at the top of their sport (or art) are
believed to actually possess this level of muscular control.