It is a form of deep breathing with various benefits, including the following:
- Warming the breath so that our body readily absorbs the oxygen inhaled, and hence increase the oxygen in the blood
- Build up our stamina by lengthening our breath and creating the rhythm for our sun salutation
- Deepen our asanas
- Detoxify our body
The first time I was taught, my teacher told me that it is also called the ocean breath as it sounded like the waves crashing on the beach. I always like to joke with my students that it sounds like Darth Vader (from Stars War). During my 200-hour yoga teacher training, I often feel giddy during sun salutation at the initial stage. With the practice of Ojjayi breath, I was able to regulate my breath and feel less of the giddiness. I've also used Ojjayi breath whenever I feel cold and it would warm up my palms quite quickly.
The article below summarises the benefits and way to master the Ojjayi breath, for your reading pleasure:
http://www.chopra.com/articles/learn-the-ujjayi-breath-an-ancient-yogic-breathing-technique