Get down on hands and knees, with hands placed directly below your shoulders, your knees under your hips, and your toes pointed into the ground.
Take right hand and place it behind your head. While keeping your lower back still, drive your right elbow up toward the ceiling. You should feel some tightness as you open up your chest. Pause for a second at the top then reverse the motion, driving your right elbow under your torso, into the space between your left arm and your left leg. Go as far as you can without moving your lower back.
That's one rep. Repeat for given reps and switch sides.