When stressed, your heart rhythm becomes chaotic (incoherent). This signals danger to the brain. To regain control, you must align your breath with your heart rate to enter a state of 'Coherence'.
Simple, effective, and guided.
Inhale smoothly for 5 seconds. This stimulates the sympathetic system slightly (acceleration).
Exhale smoothly for 5 seconds. This stimulates the parasympathetic system (braking).
Doing this for 5 minutes puts your heart into 'Resonance Frequency' (0.1Hz).