Both techniques, ujjayi and sitali, involve narrowing the glottis. This action enables you to stretch, lengthen, and regulate your breath. With sitali you can experience the inhalation as a pull; draw the breath in with a suction like action, a sipping in of nectar.
From the Shiva Samhita
"When the skillful Yogi, knowing the laws of the action of prana and apana vayus, can drink the air through the contraction of the mouth, in the form of a crow bill, he's entitled to liberation. The wise Yogi, who drinks the ambrosial air, destroys fatigue, decay, old age, and injuries."