It took me a long time to learn how to do this. Mostly because I was scared of being upside down, and also because I just didn't have the core and upper body strength to balance for more than half a second. I've finally got it down, and it's become one of my favorite inversions. It strengthens the lungs, builds core, shoulder, forearm and tricep strength. It also improves digestion, and helps to relieve stress and mild depression.
Do not do this pose if you have a headache, have high blood pressure, a neck or back injury, a heart condition or are menstruating.
TIP: Most of the weight should be in the forearms. There should be very, very little weight in the head/neck. Practice against a wall first.
PS- A pose that's great for hamstrings and abs, a pose that improves posture, and a strength-building pose we see often in yoga.