Setup and Key Actions of Half Frog Pose
Start on your belly, in sphinx pose (up on your forearms with your elbows stacked directly under your shoulders). Engage your low belly, broaden your collarbones, and lift up through your chest.
Then, come up onto your left fingertips, walking your left hand out wide (so wide that it comes off of your mat and onto the floor. Lightly squeeze the back of your left thigh and the left side of your butt as you bend your left knee and draw your heel in toward your seat. Catch hold of the big-toe side of your foot with your left hand, bending your elbow to draw your heel in toward you for the thigh stretch. Continue to engage your low belly, and keep your rib cage and chest facing forward, toward the short edge of your mat.
Come out of half frog pose if you feel any pinching or pain in your lower back or shoulders. Otherwise, remain in the pose for a few breaths. For a deeper shoulder stretch, play with "switching your grip"—spinning your left fingers forward to point toward the top of your mat, and your left elbow to point up toward the ceiling. Press your right forearm into the floor, as though you were pushing it away from you, continuing to lift up through the pose. You can even straighten your right arm if it's comfortable to do so.
Modifications of Half Frog Pose
For a gentler variation, instead of coming into the thigh stretch from sphinx pose, begin lying on your belly with your right forearm resting on the floor in front of you, and your forehead resting on the back on your right hand.