Dean Lerner's reply:
Dear Cheong Weng Kit,
As the English name correctly implies, the weight should be borne on the head in Sirsasana. The instruction to take only a certain percentage of the weight on the head has been around a long time. In fact, I was taught this, too, more than 25 years ago. But it is wrong, and promotes unnecessary fear. Head balances done correctly not only keep the neck strong and healthy, but correct neck problems.
Let's see why. Stand in front of a mirror. Stand up straight, interlock your fingers, and place them on the back of the head as if you were doing Headstand. Extend up through the top of your head, keep your elbows in, and draw the shoulders down away from your ears. Observe the neck. If you performed these foundational actions accurately, it should be long and extended. The neck receives this length without working itself. Now, imagine you are upside down in head balance. Without lowering your forearms, take your head slightly down as if to bear only 30 percent of the weight on your head. What happened to your neck? It got shorter, more compressed. So while the idea sounds good, it actually creates the very problem it is meant to prevent.
Isaac Newton's third law of physics beautifully sums up the proper actions in Headstand: "For every action, there is an equal and opposite reaction." When you press the head and forearms down and lift the shoulders firmly up, space, extension and stability are created in the neck. Therefore, do not use a soft cushion for the head and forearms, as this will absorb the descending pressing of the forearms and prevent the upward rebound action. Use a firm blanket for the head, or a sticky mat folded four times. Lastly, learn do the pose correctly, patiently building time in it, little by little. Then you will have success with Headstand, the "king of the asanas."
Certified Advanced Iyengar instructor Dean Lerner is co-director of the Center for Well-being in Lemont, Pennsylvania and teaches workshop across the United States. He is a longtime student of B.K.S. Iyengar and served a four-year term as president of the Iyengar National Association of the United States. Known for his ability to teach yoga with clarity and precision, as well as warmth and humor, Dean has conducted teacher training classes at Feathered Pipe Ranch in Montana and other locations.