When attempting to clarify the differences between Yin and Yang Yoga we must keep in mind that these are relative terms. Something can only be categorized as Yin or Yang in comparison to another thing.
In regard to the tissues of the body the yang tissues are those that are softer, more elastic and contain more fluid, which include the muscles. The yin tissues are those that are harder, dryer and less elastic, which includes the bone, ligaments, tendons and fascia.
Yoga that emphasizes muscular contraction and repetitive movements such as Ashtanga or Vinyasa flow can be thought of as being Yang. These styles of yoga can be demanding on the muscles and joints and as such the poses are generally not held for a very long period of time.
In contrast Yin yoga emphasizes long holds of around 3-5 minutes so as to promote the lengthening of the connective tissues and an increase in the space around the joints. The muscles surrounding the target area are completely relaxed in order to facilitate this intention.
Even though many of the poses that are performed in a Yin Yoga class resemble that of Hatha Yoga, the way the poses are practiced may vary greatly. For example in most yoga styles when practicing forward bends the emphasis is on keeping the back straight and engaging the muscles of the upper body to assist with increasing the stretch along the back of the legs. In Yin Yoga these same poses will be practiced with the spine intentionally rounded and the muscles of the upper body relaxed so as to promote the optimal lengthening of the spine and the release of contraction in this area of the body.
When we discuss yoga in terms of yin and yang we must also consider that these categorizations are not absolute. There is always an aspect of yin within yang such as the experience of communion with the breath and inner clarity that may occur during an intense flowing sequence.
Likewise during the practice of Yin Yoga there will be an element of yang, which may present itself as the rhythmic flow of the breath, or the recruitment of muscles outside the target joint in order to increase the beneficial effects on the major area of focus.