As the name suggests, [this practice] involves taking two breaths in before you exhale slowly. The double inhale breath is especially helpful if you have a habit of chronically taking shallow breaths, as it will help you discover just how deeply you should and could be breathing.
This breath is also known as the physiological sigh. Sighing is an essential reflex for healthy lung function and is something we all do naturally from time to time. You may sigh during your sleep if there is an excessive build-up of carbon dioxide (your waste gas) or if you are anxious or even upset and crying.
When you sigh, you reset your breathing. Your lungs are full of millions of alveoli, which are tiny, balloon-shaped air sacs in which oxygen is swapped for carbon dioxide. Each time you inhale, your alveoli inflate. Sometimes, some of these tiny sacs collapse or dry out (from mouth breathing). The second inhale reinflates those tiny sacs, increasing your lung capacity and helping your body to remove carbon dioxide more efficiently from your system.
In sound healing, we view sighing as a form of release, too — helping to rid your body of tension and trapped emotion.
— Farzana Ali in Sound Healing