The practice of Yin Yoga comes out of the Daoist philosophy and it's focus on achieving a state of balanced energy between Yang and Yin.
Traditional Hatha yoga is considered a Yang practice, due to its focus on flowing movement and building muscle strength and flexibility.
In Yin Yoga we seek to slow down, using fewer poses and holding them for longer periods of time. This effort benefits the connective tissues of the body by strengthening tendons, joints and releasing fascia.
Yin theory also devotes focus on energy channels (meridians) within the body. These channels run through all tissues, including our organs and opening these channels can lead to reduced stress and improved health. In addition, the longer poses allow us to build our mind/body awareness; bringing us into a deeper sense of balance.