Sama Vritti Pranayama also known as Box Breathing or Equal Ratio Breathing is one of the quickest breathework practices to drop yourself into a calm state. A fast and effective way to ease panic, release worry, and get the nervous system back under control.
Pro Tip: practice while sitting in traffic or standing in line to see this beauty in action.

WHEN TO AVOID OR MODIFY EQUAL RATIO BREATHING:
Pregnant people: Should practice equal breathing without holding the breath, NEVER REATIN YOUR BREATH WHEN PREGNANT
People with high blood pressure: Should not hold their breath after inhaling 
People with low blood pressure: Should not hold their breath after exhaling 
People with lung conditions: May have difficulty taking deep breaths, and should proceed with caution 

After watching the video for the first time, you can go right into the practice by skipping to minute 1:30.