Gonzui (meaning "Soul Inhalation") is a supplementary technique used by Arrancar to devour as many souls as possible within a large radius to significantly increase their size, power, force, and spiritual energy, while being content from the devouring of souls.
Yammy is the first and so far only Arrancar shown using this technique. Humans that have spiritual awareness, or mediums, can resist the technique, even if they only have the slightest amount of spiritual power. This is shown by the Karakura High School student Tatsuki Arisawa when she resists it. Gonzui was strong enough to cause her to feel great fear, complete loss of energy and even loss of consciousness shortly after. Yammy commented after devouring at least a few dozen souls surrounding him that they tasted horrible, and Ulquiorra replied that it's to be expected from such weak Humans.