The Navy SEALs use 2 breathing techniques that force the body into a more relaxed state when they’re in a high-pressure situation:

• Tactical breathing
• Box breathing.

Is a technique to use when you feel yourself having a fight-or-flight response. It involves all your breathing muscles–from chest to belly.

• Place your right hand on your belly, pushing out with a big exhale. Then breathe in through your nostrils, slowly drawing the breath upward from your belly to your upper chest.
• Pause and exhale, starting from your chest and moving downward to the air in your belly. Imagine your belly button touching your spine.
• Once you’re comfortable with a full, deep breath, repeat it, this time making the exhale twice as long as the length of the inhale. 

It’s meant to ground you, sharpen your concentration, and leave you feeling alert but calm. 

• Push the air out of your chest, keeping your lungs empty for the count of four. Then start the tactical breathing, inhaling through your nose for a count of four, drawing air into your belly and moving up into your chest. Hold the air in your lungs for a count of four.
• The movement should feel fluid and open. Exhale smoothly, starting at the chest and moving to the belly, for four slow counts. Complete the box with a pause of four before beginning another repetition.
• Continue this technique for five minutes.