(photo courtesy of http://www.yinyoga.com)
Fascia is a structure of the connective tissue and is composed of collagens, elastics and reticular fibers. It surrounds our muscles, blood vessels and nerves. According to “The Complete Guide to Yin Yoga” by Bernie Clark, muscle is ~70% muscle cell and ~30% fascia. When we are cold in our muscles, we demand more out of our fascia, stressing the connective tissue and joints and ligaments (all interconnected) in a beneficial way in Yin Yoga. If the objective is to get the permanent physical benefits of Yin then you practice cold. If your objective is to get into a pose (asana) deeper in a yang style practice, then go warm. For example, if your peak pose is Vishvamistrasana (a very yang pose) then, you need to warm the body in your yang practice. You will be able to get down deeper, but using the warm muscle already stretch and not stressing the connective tissue as much. It comes to intention. Yes, you can stretch warm.
Think about what happens when you wake up in the morning. You feel stiff. Muscles are tight. Muscles are cold. Then you start your yin practice…marinating in a yin posture, finding your first edge and getting that first stress. You cannot get too deep while cold, our the muscle spindles react to protect your body. It’s holding and stressing for time. Gradually stressing longer. Remember that muscling into a posture is not yin. Time is the magic ingredient. Time goes on, stress is on the connective tissue, fascia, ligaments and joints. The key word I use is stress. Not stretch.
Bernie explains it so well. When I went through yin training, he explained it through showing rubber bands. Click on the link attached. The different stresses on cold versus warm. Are you going for maximized stretch? Neither is wrong but what is your objective? You can get stillness benefits of the practice if you are completely warm prior to class. It just comes down to intention.