I don’t remember my first encounter with puppy pose, but since then, it has definitely become one of my favorite restorative asanas.  It is somewhere between Adho Mukha Svanasana and Balasana: your shins and feet are flat against the floor, knees at a 90 degree angle, and your shoulders to waist are in Downward Facing Dog.  Time spent in Puppy pose reminds me to open my shoulders and stretch my lower back.  Often in Adho Mukha Svanasana, I get so wrapped up in stretching my hamstrings and tilting my tail bone that I lose the integrity of my torso.  Puppy pose gives me all the torso benefits without the added thought of hamstrings.  Also, because it is a kneeling asana in which your head is lower than your heart, it works parasympathetically on the nervous system, calming a racing mind and providing welcome oxygen to the brain.
So, to come to this lovely restorative asana, I begin in Cow Pose (palms flat on the ground, torso like a table, shins and feet flat on the ground).  I then shift my weight away from my palms, pulling back into my hips while extending my lumbar spine.  I retract my scapula and push my hands against the ground while lowering my chest.  This presses my shoulders forward, opening my tight anterior deltoids.  While in this asana, I imagine myself to be a puppy, tail wagging as an invitation to play.
I find that this asana gives me more energy and enthusiasm for whatever comes next, much like the puppies after whom this asana is named.  How can such a playful asana be anything but energizing?!