Yin Yoga contains the ancient, and some say original, form of asana practice. The sages who pioneered the 
path of yoga used asanas to strengthen the body, so that they could sit for long periods in contemplative meditation. If you have ever sat for a long time with legs crossed, you know the hips and lower back need to be strong and open. The sensations you felt were deep in the connective tissues and the joints. These are the deep yin tissues of the body, relative to the more superficial yang tissues of muscles and skin. Yin Yoga opens up these deep, dense, rarely touched areas. 

Recently Yin Yoga has been "re-discovered", and more and more students of yoga are adding this challenging form of practice to their regular sadhana. Years of a dedicated asana practice will make anyone healthier, stronger, and more flexible, but at some point the muscles will have reached their limit of flexibility. New depths in postures, deeper ranges of motion, or an increased flow of energy may only be achievable by focusing on the deeper tissues of the body. This is why so many students are finding Yin Yoga to be the perfect compliment and balance to their more active, yang practices.

 

The definition in Wikipedia:

 

Yin yoga, so named because of its correspondence to the Taoist concept of yin, is a very distinct style of Chinese yoga similar to hatha yoga that stretches the connective tissue(joints). Experientially, the practice is characterized by passive asanas held for several minutes each. Some[who?] believe that yin yoga is the oldest form of hatha yoga, since it is the ideal method of physical conditioning for prolonged meditation, which is the principal concern of the ancient Hindu practice of Raja Yoga

 

I love to teach YinYoga together with Poweryoga because it connects the Ying and Yang in every person. The best I can imagine. Namaste Christel