In general, stretching restores muscles and connective tissue back to normal resting length and improves joint mobility, it does not make muscles longer or looser. Stretching does not improve range of motion. Range of motion is dictated by Gamma-Gain, individual anatomical limits and elastic tissue genetics. Stretching does not 'relax' or 'release' muscles. Stretching does not 're-set' or 'turn-off' muscles. It is impossible to stretch 'one muscle.' There are (4) types of stretching, and each type assists in a different way.
An active stretch occurs when a person attempts to stretch a targeted area by going into a position that elongates a part of the body. Generally, gravity is doing the work to create the stretch. An example is a seated forward fold (like slouching) with the head and arms hanging. This stretch might be performed in order to 'loosen' back muscles. It is important to note that the target muscles can't ALSO provide posture during the stretch.
An active assisted stretch occurs when a person attempts to stretch a targeted area by going into a position that elongates a part of the body while adding a bit of force to aid the stretch. An example is tipping the head to the side while using the hand to push the head further in the same direction. Sometimes the targeted muscles will feel a pulling sensation with an active assisted stretch. Care should be taken to limit any overpressure and avoid pain.
A passive stretch occurs when another person has your permission to place your body into a shape that targets general or specific elongation. The person may pull (distract) or push (direct) on your body or limbs with their hands or another body part, or with the use of props. This stretch might be performed in order to get a deeper stretch than you can get by yourself. This kind of stretch requires significant knowledge about the limits of the human anatomy and the purposefulness of any stretch... not all stretches are safe. All stretching should be pain-less.
A passive assisted stretch occurs when another person has your permission to place your body into a shape that targets general or specific elongation while you attempt to use your own strength to aid in the stretch (either compliance or resistance) and may involve the use of props. This stretch gets the best overall benefits in the shortest treatment time and is the safest, but requires clinically-based education and experience and your alert cooperation.
Stretching should never produce pain. Stretching may produce some temporary discomfort, and as the therapist progresses it should feel incrementally better in real time. The therapist should 'preview' your muscle tone before stretching begins, and check it again after the therapy to notice improvement. An excellent therapist knows how to position you and stretch you without ever causing pain. It is UNWISE to get your whole body stretched in one treatment.