Yin Yoga empahsizes holding postures for 3 to 5 minutes to access the dense fibrous ligamants, or connective tissue of the joints.  All of the postures are non weight bearing, practiced in a seated postion.  The intention is to encourage circulation in less flexible areas, to lengthen and release deep cnnective tissue, especially in the joints closest to the core of the body: the hips, lower back, and the sacrum area.  As connective tissues are tough and fibrous, they stretch the best when held for a long time.