Yin Yoga works differently than more vigorous forms of yoga and targets different tissues in the body - the fascia. You've seen fascia before if you've prepared meat for eating; it's the white-ish, tougher layer surrounding the chicken breast or other portion of animal muscle. You have it, too.
Yin poses are longer and focus on releasing effort, working with intense sensation and a cooling breath, generally through the mouth. It's a very meditative practice and is great for getting ready for seated meditation as well as a good night's sleep.
from a recent email responding to someone interested in fascia:
Fascia are the layers of connective tissue covering and connecting muscles. In normal function there is fluid that lubricates the muscles moving within these sheaths. The fascia is a different kind of tissue than muscle - more like cartilage than like muscle - so it responds to a different kind of treatment and movement. Fascia is effected by long, slow forces of stretch and compression in a cool environment - think braces on teeth, while muscle responds to fast, hot, repetitive motion. My favorite analogy is that fascia respond like teeth do: when you want to the change the alignment of teeth, you don't wiggle them back and forth every day (like lifting a weight). You apply braces and make small changes over time.
In addition, the liquid that lubricates the muscles' glide within the fascia can become stagnant with sedentary lifestyles, rest after surgery or even after a night's sleep. A morning routine and proper hydration are enough to address the night's sleep, but more dedication and patience are required after weeks, months or years instead of hours.
Yin Yoga specifically targets the fascia. I use yin poses at the beginning of each of my classes and teach a whole hour of it on Friday evenings. Want to feel how your body's connected, effect transformation on another level, slow down, work on your meditation seat or break through a plateau? Yin may be just what you're looking for. Here are a few poses below.
Want more? Sign up for Inspire Newsletter at the right and email me back when you receive your first missive. I'll send you a 5 page .pdf I used in an Intro to Yoga Class. Plus, here's Bernie Clark's youtube channel, where he posts great videos. He's the founder of this non-trademarked style and teacher of and with Paul Grilley (from whom I first heard the orthodontic analogy) and Sarah Powers (from whose Yin Yoga Workshop at the 2011 SF YJ Conference I benefitted greatly).