The app allows you to see your heart rate, measured on Apple Watch, right on the paired iPhone. To get started, open iPhone app, then open Watch app or vice-versa.
Both apps will immediately connect and stream heart rate data from your wrist to an iPhone's screen.

Notice: this app cannot be used without Apple Watch.

Be carefully to allow the watch app to have Heart Rate permission. If you denied it or closed app while prompted, go to iPhone privacy settings and allow app to have access to Heart Rate.

Use this app to control your workout or cardio fitness speed without an effort.

You can also use the app to allow your buddy or coach to get known if your training is going to hard or dangerous.

Watch app works in background, so it can be used along other Watch apps, but background session is limited for an hour, so if you have a very long workout, once an hour you have to reopen the Watch app.