The hatha yoga teacher training program from which I graduated used, as its basis, the universal principles of alignment codified by John Friend in his Anusara Yoga. (You'll find a brief description of those principles at the end of this post.)
One principle that was especially inspiring was the idea of "opening to grace." As a long-time professional dancer and dance teacher, I was familiar with calling on unseen forces to help me center myself for challenging movement sequences, but in the context of yoga, this concept has an even deeper meaning for me.
To give just one example: If you are deeply connected to all of the life-force (prana) in the universe and can breathe deeply to collect more of this energy into the core of your being, you can, at every moment, have access to much more capacity for transformation, self-expression, and being than just the physical little mass of muscles, bones, and nerves that we sometimes think we are.
One of the many beauties of hatha yoga is that it challenges us, in a safe way, to use our bodies to reach the deepest levels of our hearts, minds, and spirits.
Anusara Yoga: The Universal Principles of Alignment
Open To Grace: Firmly stepping into the currents of grace with a solid foundation and a heart open to the infinite possibilities.
Muscular Energy: Drawing from the outside in, from the periphery into the focal point* of a pose. Creates strength, stability, and a full physical integration.
Inner Spiral: An expanding energy that spirals inwardly from the feet up the legs, through the top of the pelvis, away from the core. Inner Spiral helps you widen the thighs and pelvis.
Outer Spiral: A contracting energy that spirals outwardly toward the core, from the waist down through the feet. Since this energy brings the tailbone and thighs forward, the idea is to find the balance between inner and outer spiral—in every pose.
Organic Energy: Radiating energy from the inside out, starting from the focal points of the pose to the body’s periphery. Organic Energy is an expansive expression of our own true nature, which is inherently auspicious and free.
*The focal point is one of the three places you draw into and expand out from and is determined by the most weight-bearing part of the pose; 1)the core of the pelvis (legs, torso); 2) the bottom of the heart (arms); or 3) the upper palate (head).
Here's more on the concept of loops and spirals.