Before becoming a regular at my local yoga studio, I always assumed those rectangular blocks were there to help you balance in challenging poses. Yet, as I tried different classes, I learned this versatile prop can be instrumental in getting an easy but deep stretch and easing tension in your back and hips. You can literally just lie down on them for a few minutes and start feeling relief. According to Jennifer Purwin , a former yoga teacher and physical therapist who specializes in pelvic health and orthopedics at FemFirstHealth in Manhattan, the relief you feel when lying on yoga blocks (or other yoga props) comes from your muscles being externally supported and able to relax, allowing gravity to do the heavy work.  “That might be a block underneath the thorax or the ribcage, one under your head, maybe on the outsides of your knees.…