Yin yoga is based upon the Foundation of ancient practices of Yoga. The 'original yoga poses' are all floor-based, passive postures as depicted on stone and scripture in Ancient Kemet and Asia, however, the development of the practice plus western adaptations have resulted in thousands of postures (asana) that are generally active (yang) and are practiced much more commonly.
A yang practice is beneficial as it helps to make and keep the body and mind strong, and it is very aligned with the active Western mindset, however, societally, we are naturally less inclined to practice Yin cultivating activities which eventually leads to physical, mental, emotional and energetic imbalances.
Yin cultivates a state of being, not doing, and teaches us to surrender to what IS, in each moment, in all senses of the word. On a physical level, we are able to nourish the deep tissues and joints by intentionally resting in different 'shapes' for a minimum of 1 minute. Each shape targets particular areas of the body and energy system/meridian/chakra and provides ample opportunity to explore the physical, mental emotional, and energetic self throughout.
It forms part of the discipline of yoga that is often neglected and therefore, though it may seem 'easy' due to the passive nature, it can actually be quite challenging as we are (for the most part) programmed to be active.