The Inner Spiral of the wrist vs the Outer Spiral of the Shoulder
When students first begin to practice Downward Facing Dog, one of two common misalignments typically occurs in the upper body:
As teachers, most of us probably spend a lot of time telling students to ground the inner heels of their hands and an equal amount of time telling them to work the external rotation of their arms at the shoulder joints, while performing corresponding hands-on assists that encourage these actions. These cues will help for awhile, but over time there is the potential that many students will take the "externally rotate the shoulders" cue too far, causing biomechanical stress/torque at the shoulder, elbow and wrist joints.
In the following video...
In both Agnisambhasana (Firelog pose, or Ankle-to-Knee pose) and Gomuhkasana (Cow-Faced pose), one of the key anatomical factors is that the thighs need to laterally/externally rotate in order to keep the knees safe. If the thighs don't laterally rotate enough in these postures then the medial condyle of the femur will dig into the medial meniscus of the knee and overstretch the lateral collateral ligament (for a more detailed overview of this, be sure to check out the video Keeping the Front Knee Safe in Pigeon). A great hands-on assist in both postures is to manually press the outer thighs down to help facilitate the lateral rotation. For more flexible people, you can also deepen the stretch in these postures by adding a little bit of your body weight to the top of their pelvis to increase anterior tilt. Check out the video to see these assists, and then practice them with a friend or in class!