Heart rate is not steady even at rest. It speeds and slows with each breath, and slow breathing practices work directly on that link.
The heart has a brake as well as an accelerator
Two branches of the autonomic nervous system reach the heart's pacemaker. The sympathetic branch increases rate, and the parasympathetic branch, via the vagus nerve, reduces it.
At rest the vagal input dominates, holding the pacemaker below the rate it would run at unopposed.
Changes in heart rate over short intervals mostly reflect this brake being applied and released, because vagal signalling acts within a single beat while sympathetic effects build more slowly.
Breathing modulates the brake rhythmically
During inhalation, vagal output to the heart is briefly reduced and heart rate rises. During exhalation it is restored and heart rate falls.
This oscillation is called respiratory sinus arrhythmia, and it is present in healthy people rather than being a fault.
Its size depends on breathing rate and depth, which is what makes it responsive to deliberate control.
Slower breathing produces larger swings
At ordinary breathing rates the cycle is short relative to the delays in the reflex loop, so the swing is modest.
Breathing slowly, in the region of six cycles per minute, brings the respiratory rhythm close to the natural frequency of the blood pressure regulation loop.
The two oscillations align, reinforcing one another, and the beat-to-beat variation becomes considerably larger. This is the physiological basis of the breathing rates taught in many practices.
Pressure sensors are the other half of the loop
Baroreceptors in the walls of large arteries detect stretch and report blood pressure continuously.
When pressure rises they increase vagal output, slowing the heart; when it falls they do the reverse. Breathing changes chest pressure and therefore blood return to the heart, feeding this loop.
Slow breathing engages both pathways at once, which is why the effect is larger than either alone would produce.
What the measurement does and does not show
Heart rate variability metrics quantify this beat-to-beat variation, and they respond within a session of slow breathing.
The measure is sensitive to posture, time of day, illness, alcohol and recent exertion, so single readings are noisy and comparisons between people are of limited use.
Slow breathing can also cause lightheadedness if it becomes overbreathing, and anyone with a cardiac or respiratory condition should ask a clinician before adopting a structured practice.