Ujjayi pranayama is so called breath because of the ocean sound produced and is sometimes rather flippantly referred to as "Darth Vader" breath. And even though all pranayamas are done through the nose, it is helpful to begin practicing this one through the mouth.
It involves contracting the glottis while breathing to make a vocal "Ahhhhh" sound. This is where the name of the breath comes from: the sound of the ocean.
This pranayama helps clear the nasal passages and enhances the functioning of the thyroid gland.
It is particularly beneficial in respiratory disorders like sinusitis, bronchitis and asthma. However, those suffering from High BP are advised not to practice this.