This pose is the counter-pose for, setu bandhasana, the bridge pose. In Bridge we squeeze the throat in a deep forward bend, massaging the thyroid and lengthening the neck muscles. In Fish the throat and thyroid are opened along with the chest in a nice back-bending pose.
To achieve this pose: Lie on your back with legs together and enough muscle activity to keep the knees pointing up. Place your hands, palms down, under your body with wrists together and elbows straight. On an inhalation, press your elbows down into the mat and lift your body off the floor. You can bring your head back and rest on the crown or the back- whichever is best for your neck. Hold for 5-8 breaths or half the time you held the bridge. Rest in savasana (relaxation on your back) pose before moving on.
It may take a few tries to get off the ground but the benefits of this pose are well worth the effort. Enjoy!