Breath of Fire is a foundational technique.
It's a rapid inhale and exhale through both nostrils with no pause in between. It comes form the navel point and solar plexus.
How to do it?
Exhale powerfully through the nose by pressing the navel point back toward the spine. Relax the muscles to inhale. The inhale and the exhale are the same length.
It's like trying to blow a candle with your nose.
Ideally it should be 2-3 cycles per second, but start at your own pace.
Start your practice with 30 sec to 1 minute and build it to 3 minutes daily.
Don't do it if your pregnant or menstruating.
If you feel dizzy, take a break and drink lots of water.
If you're trying to quit smoking, do breath of fire every time you want to light a cigarette.