How to: Roll up a towel and place it under the bottom third of your shoulder blades, then like back with knees bent to 90 degrees, feet flat (a). (dailyburn.com)
Keeping the right knee bent at 90 degrees with weight in your right heel, bend the back left knee, sliding it forward and then extending it straight back behind you again. (pumpsandiron.com)
Practice keeping your body and thighs closer to vertical by keeping knees bent. (mindbodygreen.com)