Posterior Capsular Stretch
Pictured: Standing version of what is described below
Shoulder injuries are common when doing strength training and one way to decrease your chances of injury to this area is by, you guessed it, stretching the muscles.
This stretch can reduce the risk of shoulder injury by improving flexibility in the rotator cuff and posterior capsule and improving range of motion in the shoulder.
How to do it: Lie on your right side with knees bent. Bring your right arm to a 90-degree position out from your body. Use your top arm to slowly draw your right hand toward the floor. Make sure to apply a small amount of pressure to the arm, do not force range of motion. It’s common to roll backward onto the shoulder blade, so make sure you stay directly on the shoulder. Take deep breaths the entire time. Repeat on the other side.
Please find another image for downward facing dog. The model is making a pretty standard error that folks new to yoga do–sacrificing a long spine from neck through tailbone (which makes for a long line from hands to tailbone) in order to have straight legs and feet flat on the mat. Your instructions are correct in that they point out that there might be a slight bend to the knees, but they don’t explain that this should be done if needed to keep a log spine, and that while the heels *reach* for the mat, they may never get there.