Breath of Fire is one of the foundational pranayams of Kundalini Yoga, and you will find many yoga set write ups that contains this type of breath.
It is a fast and steady breath through the nostrils with the mouth closed, unless stated otherwise. The inhalation and exhalation is kept equal and the breath is continuous, with no pause between them.
Breath of Fire done for more than 7 1/2 minutes is said to completely cleanse the blood.
It gives a strong sense of rhythm which shows up as being more intune with life.
Breath of Fire is usually practiced to gain more energy but also works adaptogenicly, and can be very relaxing too.
Make sure you stay relaxed with your body and don’t tense up or reverse the breath (paradoxical breathing). The focus is on your nostrils and on breathing. The navel will move but finding the rhythm is usually easier if the focus on the navel is not over empathized.
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