Practice often takes the following order (but no need to impose any strict rules certainly if it gets in the way of practice,limits it):-
2.Pranayama -or Breath Control
So you move from gross to subtler still…
Pranayam might precede asana if you’ve got a friskier or vigourous asana practice which might takes more centre stage perhaps.And doin it can prepare for asan–yes i have experienced this and would agree,that’s why there are no rulees and can depend but a full-scope yoga practice often moves from grossest to subtlest.
The pranayam is designed to cleanse and purify the nadis,the energy or pathways within the subtle energy body. Nadi shuddi.A prelimary for awaknening kundalini, which can take several years BTW ,rather than overnight although it’s possible but unsafe.
It is most potent when combined with ther limbs.Knmbhaka or retention used in breath control is a big kundalini stimulator.
Breath through both nostrils. Focus on the flow at the nostrils,the bridge ,the top,the entrance,the brow etc.
Take a big full deep breaths.
Nadi shodana or Anuloma Viloma aka alternate nostril breathing.This is a good one which you can do with your mind;that also thus makes it more potent.
Breath defines asana. In ashtanga vinyasa for e.g ujjayii is promoted.One of effect of ujjayyi is to lengthen the breath through partial obstruction of the epigottis.
Prana is controlled by Mind , and prana-control has the side-effect of calming down mind.And thus can also help mind go one-pointed.
There are no rules about the order i gave above- that is the common recipe.But you might be best doing a short & gentle asana routine or sequence that is calming rather than agitating.So you enter in a relaxed state of mind. One way to prepare is to have a decent enough savasana.And perform nasal irrigiation or jala neti beeforehand only if you can fit it in.
When i practiced alot of asanas something i used to do and for which my asana teacher at the time did was suggested to students once they got into savasana was to tune or re-tune back into their ujjayii breath,before finally letting go & relaxing and sinking into savasana.
Some of the vigourous asana practices may sometimes be ill-suited to a ful-scope yoga routine the incorporates the pranayam and meditation that follow.
Some however may do jala neti and then some light kapalabhati to clear the nasal passages and cleanse the frontal lobes,sinuses ,third-eye etc.I would just focus on light ful breathing or maybe alternate nostril breathing.There are no rules.But some pranayamas can have more specific effects especilly when you combine your practices into one integrated whole.
The ratios and retentions ‘potentize’ pranayama but i would start off,at least, with equal (relaxed) breathing, as holding the breath that long might be a strain.The ratio Siva posted is correct btw,1:4:2 Inhale:Retain:Exhale with the retention or kunbhaka on the 4 count. I would’nt worry about this at least straight out the gate as you are introducing more potency and difficulty to the practice.In fact i rarely practice like this. But might give it a go more often perhaps. There is just so many practices out there, and ways of modifying it. I tend to go with my intuition,inner guide.The out-breath is generally a longer.
For example when i do alternate nostril i commonly do equal inhale;exhal, with next to no retention.Trust the signs.Adding Kumbhaka reallly does make the pranayama way more powerful.The strictness of the ratios is probably there just for one reason to offset and balnce the fact that the unclusion of kunbhaka makes it more powerful.So the effects are balanced and the distribution of prana through the nadis is equal.
So ratios togther with kumbhaka make pranayam more exotic and powerful.I have read som explanation that the prana flows more easily when you create an oxygen deficit.
let’s say you do alternate nostril breathing,while you are breathing through your right nostril and not your left, in theory prana flows into the left-brain during that period of oxygen deprivation on a gross level ,at least, and a waking up occurs in certain brain regions and areas of the nervou system, psychic and physical.
A main effect of pranayam is to culture the nervous system.Pranayama can do this( as can other components )