This is something Ramaswami also stresses and he wrote an article in one of his newsletters that I've attached below, but first some videos.
In Vinyasa Krama we have many variations to explore while inverted, I remember Ramaswami telling a story about his teacher, Krishnamacharya. I think Krishnamacharya was in his 70's or 80's at the time, he mentioned to Ramaswami that there were 32 variations of headstand. Ramaswami, it seems looked skeptical enough for Krishnamacharya to get up and demonstrate them all (I'll try and hunt down the story. UPDATE: Madhu just added the actual story to the comments section, it was from Mohan not Ramaswami, my apologies to both).
This video gives an idea of the shoulderstand prep, which you can simplify a little, and some of the Shoulderstand and Headstand variations available in Vinyasa Krama. Useful too perhaps if you have an injury and can't do your full Ashtanga practice for a while.
And here, as promised ramaswami's article on the benefits and importance of inversions from his Aug 2009 newsletter.
HEAD AND SHOULDERS ABOVE ……
The two important inversion poses, Sirasasana and its better half
Sarvangasana, called the King and Queen of yogasanas are a unique
contribution of Yoga towards physical culture and physical therapy.
Several contemporary yogis have disputed the place of these poses and
have claimed that they perhaps are later day inventions. But in
Hatayoga they are considered as viparita karani mudras.
Hatayogapradipika refers to inversions as follows
“ There is a wonderful karana or procedure which helps to starve the
sun,( here the gastric fire). One may learn it only from a Guru, and
not from the books. If the position of the sun(stomach) is above and
the moon(the head) below (i.e., upside down) it is called
viparitakarani(inversion). Learn it from a Guru “
The pelvic area—kandasthana-, according to some yogis is a breeding
ground for many ailments. It is also the area from where 72.000 nadis
are said to emanate and also Kundalini. This area should be kept
clean. The dross should be burnt and blown away, figuratively
speaking. How does the Yogi do it?
We have an air principle in that area which is Apana Vayu. We have
also the fire principle in us in the abdominal area in the form of
gastric fire or Jataraagni. This flame is flowing upwards and in the
normal upright position the gastric fire is above the pelvic area,
flowing upward, sometimes when overactive, produces a burning
sensation in the esophagus producing the typical ‘heart burn”. The
Yogi by resorting to the inversions, as Headstand and Sarvangasana, is
able to place the pelvic area above the gastric area. Now the gastric
fire or jataragni,-- figuratively speaking—flows towards the pelvic
area and heats and purifies the Nadias and the Kandasthan, arouses the
Kundalini with the heat. The fire is further supposed to be fanned and
intensified by directing the air tatwa or apana by Mula bandh; it
draws the apana closer to the fire principle and thereby the apana air
also becomes hotter and in turn melts away the dross of the
kandasthana and arouses the sleeping kundalini. So headstand and
shoulderstand, the mulabandha and the intense gastric fire help to
cleanse the nadis and the rogasthana or the disease prone area is
cleaned and spruced up.
There is another interesting concept associated with the inversions of
which I may have referred to in one of the earlier letters/articles.
It is said that our head contains a liquid called amrita which may be
translated as nectar. This nectar gives us life and drips drop by drop
through the uvula into the stomach where it is consumed by the gastric
fire to provide the life energy to live. This reservoir of nectar is
slowly used up and with its total depletion comes the end of one’s
life. The Yogi tries to ration the flow of the nectar, by remaining in
inverted position for a length of time every day—say between half an
hour to an hour or so. During the period of time the yogi is in head
stand and shoulder stand, the amrita remains stored in the head
without dripping down.
The Hatayogapradika has this to say
The Hatayogapradipika explains the inversion mudra as follows. “The
cool nectar that flows from the moon (here the head) is swallowed by
the hot sun (the gastric fire). Hence one’s body becomes aged. There
is a wonderful karana or procedure which helps to starve the sun,
(here the gastric fire). One may learn it only from a Guru, and not
from the books. If the position of the sun is above and the moon below
(i.e., upside down) it is called viparitakarani(inversion). Learn it
from a Guru. Do abhyaa of this inverted pose and increase the duration
every day. One who practices this for a yaama (3 hrs) daily will
conquer death”. When I was young I came across a Yogi who was said to
be practising sirsasana for three hours every day. His face had a
unique bluish tinge. He also practised Mouna or silence.
So by this daily practice, the Yogi is able to increase, so to say,
his/her lifespan by 5%, or say between 3 to 5 years. Normally after
Headstand the yogi is supposed to spend equal time in shoulder stand
as well. In shoulder stand, amrita while still confined to the skull/
brain portion, now is allowed to flow to the entire head portion above
the neck and nourish all the sensitive sense organs, the two eyes, the
two ears, the mouth and the nose (shanmukha). This is also considered
necessary to maintain the acuity of the sense organs
as they are way up in the body and may not get the full nourishment .
Sarvangasana therefore is considered good for the sense organs whereas
the headstand is good for the brain.
The normal upright position and the chin up position in which we keep
our head, both result in a wasteful free flow of the limited amrita in
the head down the uvula to the gastric fire, like a free flowing tap.
The Yogis found it necessary to constantly control the flow of this
nectar and even temporarily stop it. They developed a simple technique
called Jalandhara bandha to temporarily stop and control the flow. The
term Jalandhara-bandha itself indicates the effect it is said to
produce. Jala means water and here it refers to the amrita or nectar
which is said to be in the liquid form. Dhara is to hold, here holding
the amrita in the head itself and bandha is the lock, the procedure
which helps to achieve the holding operation. So Jalandharabandha
means the lock that enables holding the nectar in the head. Of course
while we do asanas and pranayama we adjust the bandha in such a way
that we allow only a small and necessary amount of amrita to flow and
also maintain a good ujjayi control over the breath. That is why the
default position of the head in asana practice whether it is tadasana
or the seated Padmasana or Vajrasana is the head down position. One
could see the pictures of my Guru doing asanas and one could see his
head down position in most of them—even in asanas like urdhvamukha
svanasana or the well known upward facing dog pose. In the entire
vinyasakrama one would find the relaxed default head down position is
resorted to control the flow of amrita and the ujjayi breath.
Some contemporary yogis may read these metaphorical narrations with a
wry smile. However these 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.
What is equally important is that Sirsasana helps improve circulation
of the cerebro spinal fluid, which is helpful to the brain and also
for the spinal nerve bundles—the chakras. Because of the increased
pressure in the brain due to this fluid, the pituitary secretions
increase helping the better functioning of the sympathetic nervous
system which will help in many ways including the dilatation of the
bronchial tubes giving great relief to asthmatics. There is draining
of the bronchial tubes, giving some welcome relief for those with
chronic chest congestion. Many feel increased memory power and
general better brain capacity. There are cases of even some correction
of the eyesight. The vinyasas like the twists, Akunchanasana, the
backbends like Viparitadandasana in Sirsasana and Uttanamayurasana in
Sarvangasana help the spine considerably, by not only maintaining the
flexibility of this structure but also nourish the nadis and chakras
or nerve fibers and nerve bundles in the spinal chord.
In the inversions, as mentioned in earlier articles, the internal
organs get positional correction. Pregnant yoginis may find the
inversions help relieve pelvic congestion, oedema of the legs,
conditions that are prevalent during pregnancy. Practising the
inverted poses with the variety of vinyasas gives a complete massage
to all the muscles, organs and considerably increases the blood
circulation. Perhaps equally important is the effect of the twin poses
on the major joints-- the ankles, the knees, the hips and the spine.
The intra-articular space within the joints improves and hence the
joint movements when one does the various vinyasas also will improve.
Dorsal and plantar flexions performed in the ankle joints while in
these asanas help the ankles significantly. Asanas like Akunchanasana
in inversions give good relief to the knees, while inversions help
to open the hips by dragging the big pelvic girdle down a bit and
giving more space for the femur to move and rotate nicely within the
hip socket(pl refer to Complete Book of Vinyasa Yoga for headstand and
shoulder stand vinyasas). Perhaps the most benefit accrues to the
entire spine. The inter-vertebral space is enhanced and person who
practises these inversions and the vinyasas like akunchanasana and
backbends will find the spine stretching nicely and becoming more
flexible. The narrowing of the inter-vertebral space can be tackled
positively and the low back pain reduces significantly. I would say
that the inversions are the best yoga postures to alleviate low back
pain. Overall these inversions and the vinyasas in them help to keep
the spine supple and strong. It is said one is as old as the condition
of the spine. Further, because of the relaxation of the lower
extremities Sarvangasana is a good pose to help overcome insomnia.
These twin poses are very good for health.
Contemporary Yogis find the other important inversion, viz., the
Handstand or Vipritvrukshasna very popular. This is a great pose, with
a number of variations possible. However since the head is not fixed
in this group of poses, some of the finer aspects of the other two
head- fixed inversions (sarvangasana and sirshasana) may be missing.
One finds it more difficult to maintain balance and also stay for a
sufficiently long time in viparitavrikshasana or inverted tree pose
(Hand Stand) and other similar poses like scorpion pose etc. These two
regal poses stand ‘head and shoulders’ above the rest in conferring
health benefits to the yogabhyasis.