Muscle relaxation is to loosen up tense muscles by slowly moving inward in a specific order and frequency to ease tension. Full-body relaxation is to free the whole body from any unnecessary burden: extra food, fats, urine, stool, puss, salts, sugars, toxins and stress hormones, till it's light inside and outside. Both full and partial relaxation mutually affect each other. The light mode is the best for the human body, most of our day and life that is already burdened and slowed down by other physical needs and basic instincts. It saves much energy and spares us needless tension and fight with non-existent enemies.
Buddhistically speaking, stillness is the ideal mode for the body many aspire to reach, allowing maximum use of brain energy; yet, in practice this becomes very challenging, that we must make compromises. Staying still while doing many activities requiring movement is impossible. Instead, we slow down to keep somehow close to that superior still mode. But even then, some of such activities must be done fast, where neither slowness nor stillness is practical, that we have only one choice: to move with minimum tension, even when running at maximum speed, i.e. "light & fast" as these two modes naturally match, with the former making the latter less strenuous to muscles and organs. In nature, lightness and swiftness go together: birds are generally faster than mammals; e.g. the Indian swift, the fastest bird, is faster than cheetah, the fastest mammal, etc. We can't be fast when we are heavy, carrying heavy objects, or surrounded by heavy people.
We lose contact with our sense of touching, when other sensual and mental activities interfere, hampering us from feeling the tense organs that need to be relaxed. A common mistake people make is when they try to relax, they focus much on how they look, rather than how they feel, innocently trying to recall or memorize the different movements they are asked to do. This is why it's more common in dancing, which requires mental, audiovisual, and sensual focus simultaneously. Visual interference also happens when we pay attention to the "meaning" of a movement, treating it as a body language, although it's only meant for relaxation. Thus, you end up "analyzing" movements: this is funny, that is silly, scary, sexy, feminine, masculine, etc.
Other than the physical challenge, there is yet the problem of social acceptance. Loosening up can be ridiculed and even criticized by some people. Not surprisingly, the same people who do not embrace pacifism or non-resistance, in politics, happen to underestimate physical non-resistance too, i.e. lightness or looseness. In primitive patriarchal societies, where many people opt to violence and tribalism in solving their problems, men and women can equally get criticized, even punished for being "loose": loose men are considered effeminate, and loose women promiscuous. Not only loose movement, but, going to extremes, they criticize light clothes, mellow voice, laughing, crying, complaining ... living. Statistics show that obesity is highest among women in conservative societies where they are forced to hide their whole body. Social fetters lead to physical ones, and burdens lead to burdens, all to sadly burden the mind.
However, one should be careful not to turn the act of loosening up into a burden itself—into an obsession, by turning finicky and paranoid about petty everyday issues. Looseness should be natural, and thinking of it should be minimal. (It's all about minimal effort and energy-saving!)
Many of us forget to loosen up and relax when we need it most. Our body movement and tension level can get out of control and unnecessarily spoil our mood, even without our permission or desire. Thus, we should constantly watch both our motion and stillness, preventing any unwanted tension from sneaking into our muscles, and any wrong movement, extra speed, loud voice, over-reaction ... from happening, to save our energy.
Physical tension leads to mental tension, hampering the brain from lucid rational thinking, which affects all aspects of life. Some muscles get tense quickly, before we even notice them, that we have to watch and relax them quickly too, especially those of face, tongue and fingers. The tension or relaxation of facial muscles is the one most affecting our mood, and most challenging to control too. The muscles around the eyes and mouth are genetically designed to respond faster than any other muscle to inner and outer stimuli, e.g. a danger, before we even have time to curb it. We shouldn't let those muscles act on their own; rather, through training and changing one's habits, we can easily control the smallest, weakest muscle, and stop undesirable feelings from developing.
Not only our body and mind benefit from relaxation, but other people and objects too. A relaxed sensitive body gently treats objects — not unnecessarily slamming doors, squeezing handles, breaking keys, tearing cloth, paper (or skin), etc. — thus making objects live longer and function better, rather than prematurely expire. Touch-sensitive technology is more accepted in body-sensitive cultures.
Some jobs require high physical sensitivity for best performance and safety, like performing a surgery, driving a car/train/plane, or just (a waiter) carrying a tray or (a parent) carrying a baby, where stress/anger can seriously affect one's life and others'.
A relaxed body won't hit into strangers, embarrassing us and forcing us to constantly apologize for the body we can't control. It will lightly move between crowds without encroaching on others' turns or lanes. We will master body language and express affection properly, performing better socially and sexually.
Relaxation health benefits are too many to mention here, that you may find tomes written on elsewhere.
Relaxation is to hug pain with pleasure, in the form of an opposite pain, that feels as a pleasure by comparison. (Happiness is a degree of pain, and pain is a degree of happiness.) You go to every point of tension stored in your body, and carefully, slowly create opposite tension in the opposite direction, until tension builds up there too, then you tense up the parts you already loosened up ... and so on, in an enjoyable cycle of relief & tension.
Visually speaking, relaxation is moving between body ends and centers, back and forth: from fingertips/toes, to wrests/ankles, to elbows/knees, to shoulders/pelvis ... to abdomen, then vice versa. You end up looking like drawing endless "invisible circles" with your body.
One should focus on key points where tension is locked up: armpits, lower-back, center-back, lower neck; perineum, tongue, areas around mouth and eyes, areas between toes and fingers, wrests, ankles ... and most joints, where neural intersections get congested sometimes. To free the tension therein, one can do the following:
Arms & Legs
3. Muscle Tension
Power exercises can be used for relaxation, with limits. They are more stimulating than relaxing; we feel deep hour-long relaxation only after they are finished. However, we can mix few short power exercises during stretching or other relaxation techniques, for quick stimulation. After bad tension is gone from now relaxed muscles, we start creating good tension in them, gradually. Some muscles are more stimulating than others, that we should focus on, as they transmit stimuli fastest to the rest of the body. (More on good tension.)
There are many useful positions to learn, the most rewarding being the lotus position, and the upside-down position: the famous yoga asana increasing blood flow to the brain and boosting one's mood. These two positions should be moved to and from, gradually and carefully. However, other easier positions can also be rewarding: half-lotus, Burmese, squatting, sitting-on-heels, free fall, etc.
5. Mind Relaxation
For mental relaxation, one should free the mind from worries and other negative thoughts before exercise, while evoking positive relaxing thoughts. Keeping privacy if needed where you exercise is also important. (More on mind relaxation.)
Stress is personal, just like many types of pain: we can only ask others for guidance, not for relieving our pain. No one feels your pleasure or pain better than you. Thus, it's better to handle both on your own.
There are countless books written on relaxation techniques and therapy, and writers and therapists (or charlatans) who make their living from them. You can be your own therapist, because no one knows better where it hurts or pleases most, than you. All relaxation techniques are mere variations on a basic theme: "slowing down with pleasure." It has three main variations:
One must be careful not to make a wrong movement, that can cause pain disrupting and ending the whole thing. Exercising "slowly and with pleasure" is the safest way to avoid accidents.
Ironically, for one to rest or be perfectly still, one should devote some time first to moving in a relaxing way, to reach stillness and restfulness. Relaxation itself is an alternation between voluntary tension and relief, motion and stillness. It can also be a combination of both, simultaneously made, where some body parts become tense, while others relaxed.
It's better to be still and light, than still and heavy. Physical relaxation leads to stillness, and vice versa. (It's better to be still, like the lively sunlight, that is light, swift and ubiquitous, not like a still rock, that neither has nor gives life.)