Hormesis

Call it "healthy stress".  It's a fancy term for short-term pressure on a system that makes it stronger.  Cold immersion, saunas, energetic breathwork, high-intensity exercise, intermittent fasting- all of these are hormetic.  Higher doses might cause damage, but the right dose, of the right duration, is a reset signal to your body.  Hormetic activities drive your physiology to an extreme state, and allow it to recover in a controlled fashion.