What is the simplest breathing technique for stress, and does it actually work?

Slow breathing with a longer exhale than inhale has genuine physiological support — it increases vagal activity and shifts the nervous system away from the stress response. A simple version is inhaling for four counts and exhaling for six, for two or three minutes. Box breathing, four counts each for inhale, hold, exhale, hold, is equally reasonable. What matters is that the exhale is not shorter than the inhale and that the rate is slow, roughly six breaths a minute. It works best as a regular practice rather than only in crisis, and it takes minutes, not hours.