Your breathing is the only autonomic nervous system function that operates both completely subconsciously and under direct conscious control. You cannot think your heart rate down, but you can deliberately breathe it down.
The fastest verified way to eliminate acute autonomic anxiety in real-time: two quick inhales through the nose, followed by a long, relaxed exhale through the mouth. Repeat this three times to reopen collapsed lung alveoli and trigger immediate parasympathetic braking.
Used by elite tactical units and peak performers: Inhale 4 seconds, hold 4 seconds, exhale 4 seconds, hold empty 4 seconds. Repeat for four cycles to induce razor-sharp cognitive poise.