I love the standing balance postures. I love to teach them and I love to practice them. Not only do they develop total-body strength and train our use of the three primary tools of asana: breath, bandhas (energetic/muscular locks), and drishti (gaze). More importantly, they help us build confidence and focus to carry with us through the rest of our practice and, indeed, far beyond.
Truthfully, the Sanskrit name I've given for this pose -- Nirlamba Parsvakonasana -- more accurately refers to a sequence of postures otherwise known as the Side Angle Pose sequence. The sequence begins with Parsvakonasana (Side Angle Pose), then the arms bind around the thigh in Baddha Parsvakonasana (Bound Side Angle Pose), then the front leg straightens in Baddha Trikonasana (Bound Triangle Pose), and finally the back leg lifts and the body folds into this balance, the peak posture of this little flow, for which I have not uncovered a proper Sanksrit title.
This is a challenging posture for a number of reasons. Most obviously, it is a precarious balance with the arms bound behind the back. Binding not only makes the balance more difficult, but with the hands not quite available to offer fast protection from a fall, the practitioner is called upon to summon up a sense of boldness.
Physically, this pose lengthens and strengthens the hamstrings and inner thighs. It tones the feet and ankles, opens the groins, strengthens the glutes, and trains the core to work in detailed coordination. And, like any posture in which the arms are bound behind the back, this posture opens the chest by working the upper back to pull the shoulders down. This opening of the chest calls for active extension of the spine which, in this forward bend, presents a challenge to both flexibility and strength of the spinal extensors.
While nothing but practice and patience will give way to the necessary strength and flexibility this posture demands, the real trick to standing balancing postures is drishti (gaze). Fix your eyes on a single point before you come into the posture and keep the gaze there as you make the transition so that your attention, rather than darting around the room, is directed inward to watch and control all the little tweaks and adjustments that need to take place in order to remain in the posture. And don't forget to breathe. Steady breath and steady gaze make for steady balance.