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Bluetooth chest straps that use the standard Bluetooth heart-rate service. A Polar H10 is the reference; it can also stream raw ECG. A chest strap reports every heartbeat, which is what heart-rate variability figures need.

Apple Watch works for the live breathing feedback. It reports an averaged heart rate every few seconds rather than every beat, so beat-to-beat variability figures are left out rather than estimated.

Demo mode uses a simulated heart so you can explore everything without a sensor. Nothing it shows is measured from you.

1. Moisten the strap’s electrodes and put it on snugly, just below the chest muscles. Most straps only advertise once they sense skin contact.
2. In Herz, tap the sensor button at the top right of the Practice screen.
3. Choose Polar H10. Other standard straps connect through the same option.
4. Allow Bluetooth when asked, then tap your strap under Nearby straps.

You don’t need to pair it in iOS Settings first. Herz remembers the strap and reconnects to it next time. To use a different one, open the same screen and tap Forget this strap.

• Wear the strap before searching, and moisten the electrodes. A dry strap loses contact as you settle.
• Check it sits just below the chest muscles and is snug.
• Close other apps or devices that might be connected to it. Many straps talk to only one at a time.
• Make sure Bluetooth is on and that Herz is allowed to use it in iOS Settings › Privacy & Security › Bluetooth.

Only if you want to see the waveform or check a signal problem. On a Polar H10 it streams the raw signal 130 times a second, which uses noticeably more battery on both the strap and your iPhone. The figures you see come from the beat intervals the strap already sends, so it is off by default.

1. Make sure the Herz Watch app is installed. It comes with the iPhone app; if it isn’t on your watch, install it from the Watch app on your iPhone.
2. On your iPhone, tap the sensor button on the Practice screen and choose Apple Watch.
3. Unlock your watch and open Herz. Allow Health access if it asks; that is how the watch reads your heart rate.
4. Tap Begin session on your iPhone. The watch starts reading your heart rate as the session begins, and the first reading arrives a few seconds in.

Keep Herz on screen on the watch for the whole session. The assessment is the one exception to starting straight away: after you start it, your iPhone waits until your heart rate comes through from the watch before the first pace begins.

Yes. The watch has the same breathing guide and can run a session on its own, so your phone can stay in another room. When your iPhone starts a session, the watch follows it instead, and the two orbs move together.

Heart-rate variability is defined on the interval between individual beats. The watch’s optical sensor reports an averaged rate every few seconds, from which those intervals can’t be recovered. The large, slow swing driven by your breathing still comes through clearly, so Rhythm and heart-rate swing work. Variability figures need a chest strap, and the app won’t show a number it can’t honestly compute.

With haptic guidance on, your iPhone plays a continuous swell that rises through each in-breath and fades through each out-breath, so you can close your eyes and still be led. Hold the phone in your hand or rest it on your lap to feel it.

Turn it on or off in Settings › Guidance › Haptic guidance.

Not at the moment. The taps available to watch apps come with an audible beep, which is the opposite of calm, so the watch guides you with its orb. For eyes-closed practice, use the haptics on your iPhone.

In Settings › Guidance › Sound, choose Off, Ocean, River or Wind. Tapping one plays a single breath of it so you can choose by ear. The sound swells as you breathe in and eases back as you breathe out, and a singing bowl marks each turn of the breath.

Sound starts off. It plays alongside other audio and is set with your volume buttons.

It finds the breathing pace your heart responds to most. You breathe with the orb at six paces, from seven breaths a minute down to four and a half, for two minutes each, with a minute of quiet rest between them. It takes about seventeen minutes, and the strongest response becomes your training pace.

Start your first one from Find your resonance on the Practice screen. After that, take it again from Settings › Breathing pace › Re-run assessment.

• Sit comfortably somewhere you won’t be interrupted.
• Breathe into your belly, not your chest.
• Follow the orb exactly, even when it feels slow. The app measures your heart’s response to the pace it shows you; it can’t see your breathing.
• During each rest you’ll be asked how the pace felt.

Stop pacing and breathe normally. Slow breathing can do that. The button in the corner of the screen lets you report lightheadedness, air hunger or chest tightness at any time, during a pace or a rest.

Reporting lightheadedness pauses the assessment and sets that pace aside rather than counting it. You can then carry on when ready, finish here and keep the paces you have already done, or discard the whole assessment. If you feel unwell, don’t carry on.

To make the result your training pace, yes. Comparing one pace against another needs several readings inside every breath, which only a chest strap provides. You can walk the assessment with Apple Watch or in demo mode to see how it works, but those results aren’t kept as your pace.

It helps. Take it again on another day rather than twice in one sitting. The app keeps up to five results and uses the middle one, so a single run that went wrong moves your pace only a little.

Rhythm, from 0 to 100, is the share of your heart-rate movement happening at the pace you’re being given. It shows how closely your heart and the pacer are moving together; it is not a measure of health or fitness.

Heart-rate swing is how far your heart rate rises and falls across one breath, in beats per minute.

Settings › Data › Delete all data removes every session, resting baseline and assessment from the app. Entries saved to Apple Health can be removed in the Health app. See the privacy policy for what the app keeps.

No. Herz is for training and general wellbeing. It does not diagnose, treat or monitor any condition. If you have a heart condition, or notice anything unusual, talk to a doctor rather than to an app.