When my teacher first introduced me to asana practice with coordinated breath and movement, I experienced intense resistance in mind and friction in body . It felt more difficult then free movement (which it is), it felt like it was limiting my physical abilities (which initially it was, but that changed over time) and it was sometimes confusing – when to inhale and when to exhale? The confusion led to an agitated practice (my mind was racing to figure out what to do).
There is a simple rule of thumb which helps 99.9% of the time:
Movements of expansion and opening take place on the inhale
Movements of contraction and closing take place on the exhale
Apanasana is an easy asana to witness & experience this idea. On the inhale the knees move away from the chest in an opening movement. On the exhale the knees are brought in closer to the chest in a closing contraction.
Cakravakasana is a slightly more subtle example. Watch the front side of the torso – the abdominal and chest areas. On the inhale, when the back is hollowed, the front side of the body is open and expanding. On the exhale, when the back is arched, the front side of the body is closed and compressed.
You may want to try these and other asana with coordinated breathing to experience this idea of opening on the inhale and closing on the exhale. Do each posture a few times with a focus on the ideas of open and close and see how that is reflected in your movement.
Finally, to appreciate the natural alignment of this coordination between breath and movement, you can try to reverse the breathing, switch the inhale and the exhale and see what happens!
So the next time you are practicing asana and are not sure if you should be inhaling or exhaling, observe the movement. If it is an opening & expanding movement you should be inhaling. If it is a closing & contracting movement you should be exhaling.