Why are there so many excessive medical claims for hatha yoga?

This is because those who claim much have not studied anatomy and physiology and medicine. For example a question was raised if some mudra can reduce cancer cells. The interplay of cells in immunology is so complex that as physicians we need to read the subject several times. There are cells that activate other cells which in turn activate other cells which in turn activate some receptors and so on. Then there are antibodies to the receptors that prevent the action that is normally intended and then mutations on the receptors and so on. In such a complex situation its not possible to wave a magic wand ( except by God) and manipulate things as we wish. This is reality.

Must we master every single asana?

This is not necessary. There will be problems in the body even if we master all asanas. Do the best we can, leave the rest to God. As said before, karma is a potent factor in life. Our path is laid out by God. He knows what is best for us. We need to learn to tune our frequency to His broadcast.

What is Real Meditation?

Our physical yoga practices mean very little (click image for a better view).

Is Yoga useful for depression?

Not really. It can make the patients more depressed. However dynamic the practice be, yoga is not the full answer. It is a diversion like any other. Some take to reading , some to eating, some to yoga and some to music and so on. Love is the cure with counselling and friendship. Depressed people need care. The key element is taking the mind away from the object of depression. If it involves some exercise, its good in the process. But where is the motivation? I may be so depressed that I don't feel like even moving a limb! So the main factor is changing the mind, dealing with ourselves and then the motivation to surge ahead is there.