What "slow breathing" actually means
"Slow breathing" is a category. Anything below roughly ten breaths per minute qualifies. That includes 4:6 at ten BPM, 5:5 at six BPM, 6:6 at five BPM, and coherence at 5.5-5.5. All are slow. Only one is specifically at the resonance frequency.
Why 5.5 is different
5.5 breaths per minute lands at the cardiovascular resonance frequency for most adults. At that rate, HRV amplitude is maximised. Below or above that rate, HRV amplitude drops. This is why coherence produces the largest HRV response for a given practice — not because it is magic, but because it is at the resonance.
Does the difference matter
For HRV training and long-term cardiovascular practice, yes — the resonance frequency produces measurably larger responses. For beginners, gentle downshift, or for people who find 5.5 too slow, no — 4:6 lands you in the same territory with less pressure to hit an exact number.
The practical recommendation
Start with 4:6 if 5.5 feels demanding. Move to 5.5-5.5 as it becomes comfortable. Both are legitimate practices; coherence is just the more precisely targeted one.