This post is for D.:
The breath makes Ashtanga yoga to what it is. It's one of the specialities:
1. The breath can be heard, it's uddjay breathing.
2. The breath initiates the movements (not the other way round).
3. The breath gives the rhythm of the practice. Sometimes a movement is long, sometimes it's a tiny movment. It's important to move faster or slower. The movement adjusts to the breath not the other way round.
4. Inhaling is as long as exhaling. Often yoginis have the inhaling shorter. Take a timer to test it once to get a feeling for it.
Beside these qualities of the breath, we also count the breath.
There is a distinction between the vinyasa (the movements that connect the asanas) and the asanas themselves.
One says an asana has i.e. 15 vinyasas. This means 15 movements around the asana, 15 times inhaling and exhaling, from samasthitih to samasthitih.
When looking at the vinyasas one movement is either an inhaling or an exhaling, inhaling counts as 1 breath, exhaling counts as 1 breath.
When one says that an asana is held for 5 breaths, it means 1 breath means inhaling and exhaling.
Nowadays the poses are held for 5 breaths. In former times it was 8 breaths (inhaling, pause, exhaling, pause = 1 breaths)
To hold an asana longer can be helpful as the body gets exhausted and gives up resistance. This means that one can go deeper in an asana. One has to be careful of course when being longer in an asana. Injuries are often felt the day after and not during the practice.
In order to learn the correct breath counting, I recommend 2 sources:
1. Practice with the CD by Sharath, primary series. It's available on Ashtanga.com.
2. Ashtanga yoga by Lino Miele and P. Jois, is the bible here.
It is a challenge to practice comme il faut. I've sometimes difficulties to be with one inhaling in a pose. I need longer. I want to practice carefully. I know in which direction it goes.
Not to make additional breaths has advantages: no procrastination. Breaks give room for thinking like: OMG now this difficult marichyasana C.
Many yoginis lie on the back, staring to the ceiling before practicing urdhva dhanurasana. This makes the pose not easier. Just doing it is it, and this is the message.
Ashtanga yoga is also a mental exercise. The consequent counting and moving according to the counting doesn't allow much thinking, this calms the mind. This is concentration, this is focus.
A long post, I hope some questions could be cleared.
Next time I'll write about the breathing when doing the jumping forward and backward. I did some research here.