Do what thou wilt shall be the whole of the Law.
The second stage of raja yoga, is pranayama, which we often think of as 'breath control', but is really the control of the life force (prana). So while this practice starts with the breath, it is really the prana that is being manipulated, or better to say regulated, through the breath. Throughout the various oriental traditions of meditation and yoga are various approaches to this, each with their own theoretical models and concepts. The Taoist methods of Tai chi and Qi gong are good examples, particularly the latter, of pranayama, and I would recommend them as extracurricular activities.
The most basic form of breathing exercise is simply watching the breath. Either just gently following it or counting to ten with each cycle. This method can itself lead to the highest results and to be completely honest little else is strictly necessary, with the caveat that this is true for a mind already well disposed and balanced, in a healthy body capable of relaxing fully. The rest of us have to work at this, and so we have the methods of Yoga.
Modern man (and probably historical man too) has a huge number of stresses and imbalances to work through before he can really get down to business. Not all of us are called to live in a monastery, and all of us have baggage, so auxiliary exercises are necessary to prepare the initiate for meditation. Once Asana has been established, the first of these is of course pranayama.
If we peruse Liber E, A.C. gives various methods and variations on breathing exercises that the student can try out. I won't repeat or comment on these individually except to say that the student should beware of letting this practice turn into a sort of competition to try to constantly slow the breath more and more. The breath naturally slows as the body relaxes, the muscles and circulatory system open up, the blood flows more efficiently, delivering oxygen and nutrients to every part of the body, and with it the flow of prana becomes both stronger and more even. If you try to force a tensed body to slow the breath you only suffocate it, and the sign that you are doing this is that after a few cycles you are gasping for breath.
In all things the breathing, while being ideally slow and deep, should be natural and in harmony with the state of the body. Crowley talks about automatic rigidity, but in my experience at least this is only a preliminary stage early in the practice of pranayama. After this a gradual and deep relaxation occurs, much like success in asana, with which this state is essentially identical. Once automatic rigidity is obtained it should immediately be relaxed. Rigidity is the body trying to hold on to something, and in meditation we really don't want to be holding on to anything. The breath should be natural and slow, in through the nose, out through the mouth, slowing to a point before you start to feel short of breath, so initially it might be quite rapid. I would aim for an inhalation of about 10 seconds, and inhalation of about 15 initially, shortening a little if you start to feel short of breath, but allowing the body to naturally relax with each exhalation until a sort of settle liveliness, not the dead relaxation of a rock, but the living relaxation that we can see in a cat at rest.
You can also experiment with adding Kumbhakam (holding the breath) for short periods at the conclusion of either the inhalation or the exhalation. The two have different effects, although for relaxation I would recommend at the end of the exhalation where it can be held longest without strain. In the above suggested 10/15 cycle I would add perhaps 5 seconds to make 10/15/5, although you may well find that other cycles suit you better, so experiment, and listen to the body. Of course bodies differ, and even the same body will differ depending on a range of variables such as time of day, weather, stress levels and others. The job of the aspirant is to learn for herself which variables influence her practice and to adapt accordingly to maximise the benefits of the practice.
In A.'.A.'. terms, the initiate must be able to hold to a cycle of pranayama for a given period of time, traditionally one hour. Personally I don't think this is necessary and have methods available to determine whether the results of pranayama have been obtained. The real results should be a deep relaxation, steadiness and great improvement of the quality of the asana, which before was measured rather bluntly by how long the Zelator could maintain it. At this point we really want to look for quality rather than quantity, since anyone with a bit of grit can endure an hour of sitting. In pranayama, only constant and intelligent practice will bring about the sort of relaxed steadiness that is required.
These methods are difficult to describe, so I won't go into detail, but in general the examiner will sit with the student for a period of time in asana, and the latter will then begin pranayama. The resulting relaxation should come fairly quickly, after less than a dozen cycles, after which the examiner can test by gently pressing the student while in the asana, while himself completely relaxed and centered, carefully sensing movements of the automatic reflexes for signs of tension or instability. Small tremors and movements will betray the slightest tension. Of course, knowing that he is being tested will tend to cause higher levels of tension and thus pronounced responses, but this is ok since it makes for a better test of the students steadiness and relaxed state.
As was implied earlier in this post. Pranayama isn't about the breath. The breath is just the method. What is really being trained is prana, which is to say the nervous system and the energy which flows through it, which, while invisible, is perceptible through its action in the body. Where body and mind are not unified and prana is not under control, it will tend to respond to all sorts of outside influences, particularly those administered by the examiner who knows exactly what to look for. If the students pranayama isn't resulting in greater levels of relaxation, both of mind and body, these impulses will remain beyond his control and thus beyond the scope of his will. If he cannot control these then he does not have control over himself, except in the crudest of terms. Will is still a matter of deliberate mentation, and not yet part of the students being
Personally I wouldn't expect perfection in a Practicus, but I would expect a level of relaxation above that obtained by the same individual as a Zelator. While the testing itself could last only a few minutes I would be looking for signs that even if the prana is twitchy, the Practicus is aware of this and is able to respond appropriately, not by tensing, but by deepening her relaxation.
Space here does not permit a full detailed examination of the methods I use, and in any case they are liable to be misinterpreted unless shown first hand, for which one to one contact is required. I am aware that these methods may appear rather idiosyncratic, but actually they are derived from Raja Yoga as practiced and taught by Japanese students over the last 80 years under the tutelage of Nakamura Tempu, an ex-Japanese Army spy operating in China in the 1930's, who later practiced Raja Yoga in the Himalayas before returning to Japan to teach it to some of the early Aikido instructors, themselves students of zen Buddhism and Ichikukai misogi training, and familiar with methods of meditation. I use these methods because they work, are devoid of metaphysical speculation, and with practice can be taught to and learned by anyone. I would not say however that these methods should be universal, individual instructors will have their own methods. In my case, when I was a Practicus my instructor offered no such testing, but since I was already undergoing training in pranayama and relaxation and being regularly tested as part of my Aikido training. She, being a martial artist and Yoga practitioner, recognised the validity of what I was doing and its applicability to the A.'.A.'. work, and passed me on those grounds, For this reason I employ the same methods.
Love is the law, love under will.