[Apple Watch] What does the option of ‘continuous reading' mean?
'Continuous reading' means that the Apple watch will collect a heart rate reading every 5 seconds. It is similar to starting a workout on your Apple Watch. Normally, the Apple Watch will take background measurements throughout the day based on your activity. On average, that is about 5 - 15 minutes. People typically enable it when they want to get a finer-grained view of what their heart is doing -- e.g., monitoring their heart rate during certain conditions, working out, or even throughout the day to see which co-workers stress them out most!
If you choose to use "continuous mode," the sensor takes more frequent readings while in "continuous mode," which will impact battery life. Also, since the continuous mode is recognized by your watch and your iPhone's software as a workout, your activity rings on your watch will fill in when you're not actually working out.