–It is always practiced through the nostrils with mouth closed, unless stated otherwise.
–Breath of Fire is powered from the navel point and solar plexus. To exhale, the air is expelled powerfully through the nose, by pressing the navel point and solar plexus back toward the spine. This feel automatic if you contact the diaphragm rapidly.
–To inhale, the upper abdominal muscles relax, the diaphragm extends down, and the breath seems to come in as part of relaxation rather than through effort.
–The chest stays relaxed and slightly lifted throughout the breathing cycle.
–When done correctly, there should be no rigidity of hands, feet, face, or abdomen.
Sourced from : Aquarian Teacher