Matsya, the fish, was one of the incarnations of the Hindu God Vishnu, who assumed this form to save the world from the Flood. The Fish Pose Matsyasana, is the counter pose to the Shoulderstand and must always be practiced after it. Having stretched the neck and upper spine in the Shoulderstand, Plough and Bridge, you now compress them as you arch back, relieving stiffness in your neck and shoulder muscles and correcting any tendency to rounded shoulders. Holding the pose exercises the chest, tones the nerves of the neck and back, and ensures that the thyroid and parathyroid glands obtain maximum benefit from the Shoulderstand. It also expands the ribcage fully and so aids deep breathing and increases your lung capacity. You should remain in the pose for at least half the amount of time that you spent in the Shoulderstand in order to balance the stretch. 

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1. Lie down on your back with your legs straight and your feet together. Place your hands, palms down, underneath your thighs. 2. Pressing down on your elbows, inhale and arch your back, resting only a very little on the top of your head on the floor. Exhale. Breathe deeply while in the position, keeping legs and lower torso relaxed. To come out of the pose, first lift your head and place it gently back down, then release the arms.
Note: Your hands should rest palms down, side by side, and your elbows should be as close together as possible under your back.