The auto rest feature is available only for pool swimming. Your device automatically detects when you are resting, and the rest screen appears. If you rest for more than 15 seconds, the device automatically creates a rest interval. When you resume swimming, the device automatically starts a new swim interval. You can turn on the auto rest feature in the activity options (Activity Settings).
If you do not want to use the auto rest feature, you can select BACK to manually mark the beginning and end of each rest interval.