Head Stand :: Sirshasana
Even though it is called the head stand, you are not actually resting on your head but more on a tripod created by your arms. It is easily the most beneficial of all the poses because it reverses the effects of gravity and allows circulation to continue while the heart rests slightly. It also brings oxygen-rich blood to the brain.
The mastery of this pose is more a feat of concentration and of conquering your fears than of any great strength. The feeling should be just like when you are standing; perfectly aligned but upside down. I'm at the stage now where my goal is to be able to come gracefully out of the pose without dropping my feet to the ground, but gently lowering them down. Still working at that one.