Bhujangasana vs. Urdva Mukha Svanasana
Though very similar, in cobra your chest lifts, arms are extended but may stay bent. Hips and legs stay on the ground. In updog, the entire body is lifted, with the tops of the feet and the hands as the base, arms fully extended, with legs and arms doing most of the work. In both poses, feet are parallel, toes pointed and tops of the feet press firmly into the mat as the legs reach back. The spine lengthens, the crown of the head reaches upward, as the gaze looks forward. The chest opens and the collar bones widen and reach up. The lower abdomen lifts.
Bring attention to the low back. Be cautious of any pinching. Try to lengthen more then bend. Think of a banana, not a crispy taco! Experiment in both positions to feel the difference in muscles engaged. Now you know what's up dog!
Don't forget to breathe!