Here area a couple of very useful yoga articles:
Sore neck? Tense shoulders? I've directed many people to the following article by Barbara Benagh of Yoga Journal, and this article has been very helpful to me:
The article: http://www.yogajournal.com/practice/1969
Torn rotator cuff: Who hasn't had this problem - and frustrating too, here's help in this article: http://www.yogajournal.com/lifestyle/1102
Handstand - why is it so challenging, even when we know we're strong enough? The following article throws some light on the psychological aspects of this pose: http://www.yogajournal.com/practice/916
Here is an odyssey of someone trying to master it http://www.yogajournal.com/practice/2609?print=1
Breathing
Why do breathing practices in yoga? http://www.yogajournal.com/practice/219
What is the physiological basis of alternate nostril breathing: http://www.yogajournal.com/wisdom/927
Bandhas (inner contraction of a group of muscles)
An important part of yoga, but often misunderstood and incorrectly practiced:
Mula Bandha http://www.yogajournal.com/practice/2416
