Quite likely everybody knows there is no Yin without Yang. They are always relative to each other. Darkness for example is yin and light is yang; cold is yin and hot is yang; calm is yin and excited is yang. Now look at warm, which is yang compared to cold, but yin compared to hot. Following this principle there can't be Yin Yoga without Yang Yoga, so why only teach Yin? The reason is that nowadays our lives are very yang. In general we favour yang qualities like activity, excitement, ambition, entertainment and movement more than their yin counterparts.
Yin Yoga is a slow-paced form of yoga based on Hatha Yoga. The asanas, or yoga poses, are held for longer periods of time and because of that are all floor poses. In Yin Yoga you hold the poses 3 - 5 minutes. Aim is to stretch connective tissue and work on the flexibility and strength of the joints. This can only be done slowly and consciously and with stillness. As soon as people move muscles act, which is good as they protect our relatively inflexible connective tissue and joints. However to improve their functioning they have to be loaded or 'stressed'. In Yin Yoga you go into a pose to an appropriate edge (you feel sensations in the target area), then you become still and you stay for a while.
Yin Yoga is not only based on Hatha Yoga, but also on Chinese Medicine. If you've heard of acupuncture, you probably know that needles are placed in energy pathways, called meridians. A massage form like Shiatsu presses on certain points on these pathways and in Yin Yoga you shape your body in a certain way to target these pathways. By doing so the energy balance in the body will be restored.
The third foundation of Yin Yoga is mindfulness attention. This means simply that you don't let your thoughts wander whilst you are in a pose, but that you focus on what is happening in your body and mind.
Poses, meridians and mindfulness will all be addressed in a typical Yin Yoga class.
Yin Yoga poses can be adapted to your individual level with the use of soft props, modifications and replacements. So Yin Yoga is for everybody. Between the age of 25 and 30 human bodies start to need more yin activity then yang, but as you've seen above they are always relative compared to each other. Unlike in other forms of yoga the poses don't change when you become a more advanced student. What does change is the amount of time you can be in a pose and the quality of your mindfulness attention.
Apart from the general benefits of doing yoga, Yin Yoga practitioners gain stronger and more flexible connective tissue (fascia, tendons, ligaments) and joints. They also prevent illnesses by restoring balance in the different energies in their bodies, that is they prevent an unbalance to become a condition. Many Yin students notice that all kinds of ailments disappear, get less or are easier to cope with. Please find out for yourself what Yin can do for you!