Ramaswami writes concerning Sirsasana (headstand) and it's subroutines
'...inversions should be considered as unique contributions of Yoga, for health. Within the first few minutes of Sirsasana practice, the leg and thigh muscles, the gluteal muscles, relax. The chest, back, shoulders and neck muscles also relax as all these are not required to maintain the postural tone as in the upright position. It has been found that due to the relaxation of the leg muscles, the blood pressure in the legs drop to about 30mm.There is no great rush of blood to the head among the adept yogis due to auto regulation; yet the gravity helps to open up many capillaries in the brain, head and face which may otherwise remain partially closed. People with high blood pressure and retinal problems will have to be careful. However persons with mild hypertension and under control with diet, life style change and even medication could benefit from this posture if they had learnt it from early life. It appears to increase pressure on the shoulders which would result in the brain trying to reduce the blood pressure. Therefore if one would practice Sirshasana regularly for a sufficient duration, one’s pulse rate tends to reduce, thereby reducing the strain on the heart. Gradually there is a reduction in the blood pressure'.
There are several approaches to headstand, the above is the standard Vinyasa Krama approach.
If your new to headstands here is a link to a tutorial http://youtu.be/J68JvZtAHN4
Consider using a wall while building confidence ( this us useful when kicking up into headstand as you can tap off the wall - see tutorial link above)
When interlacing the fingers, engage the little fingers (pic 2)
Make a firm base with your arms, your going to press down through your arms and take most, if not all, of the weight on them rather than on your head.
Headstand is an arm balance
Walk in bringing the hips over the shoulders and even beyond them to use as a counterweight for your legs. (pic 5)
Engage moola and uddiyana bandha,
Raise the feet off the mat while exhaling and bring the legs close to the body (Pic 6)
Press down through the length of your forearms, especially your elbows as you raise your feet.
Focus on the hips or the pelvis in space.
Drop the shoulder blades down the back, this should create space stop your neck getting pinched
Stretch out through the whole length of your body, engage your legs and bring your attention to the
furthest point, your toes
Focusing the mind on the toes will control the balance when fully extended.
Come down by going back to half headstand, bring the legs to the chest
Make sure the toes are turned up so you land safely.