Nogare is karate’s “soft” breathing method. Some karateka call nogare "combat breathing," since it conceals your breathing motion, which prevents opponents from timing their attacks to make contact while you inhale. In practice, fighting mostly consists of short ibuki breaths, but nogare breathing should be used in lulls or breaks in the action to prevent fatigue and calm your nerves.
Nogare is much like normal breathing, with a quick, silent, and deep nasal inhalation, and a gradual silent exhalation through the mouth. The abdominal muscles are slightly tensed to hide the rise-and-fall of your chest, which telegraphs your breathing.
Nogare breathing prevents fatigue by encouraging air absorption. A fatigued person's “huffing and puffing” comes from a craving to inhale more air, but they must exhale to inhale again. This craving causes people to illogically push the air out of the lungs before it can be fully absorbed. By calming down and allowing oxygen to soak into your lungs, you can draw more benefit from each breath.
The calming effect of nogare breathing is essential because adrenaline bursts can cause unwanted tension and a degradation of fine motor skills, which can prove to be a liability.