Wim Hof says a breath hold after the exhale can send five times more blood to the brain and heart. On Diary of a CEO he tells Steven Bartlett that cardiologists caught it on heart films. Thirty hard breaths drop carbon dioxide, raise pH, and delay the urge to breathe. He argues modern life avoids the deep signals we were built for, cold and danger included. Leave those systems unused long enough and the body goes soft, immune system included. A breath hold lets CO2 rise again, which can widen brain vessels, trigger the dive reflex, and train tolerance to discomfort. Some work also links the broader method to shifts in stress hormones and immune signaling. Comfort trains the shallow layer. The older machinery only wakes under stress.