If you enable these features, the Nymi Band can track the amount of steps you take while you wear it and send those steps to Apple Health or Google Fit. For privacy reasons, these features are off by default.
To enable Fitness Tracking in the Nymi Band app, go the Services Settings section. After enabling Fitness Tracking, you will see your daily step count shown in the app's dashboard.
By allowing access to Apple Health or Google Fit (this is optional), you are adding the Nymi Band as a source of steps to this fitness service. The steps tracked by your phone and Nymi Band will not be double-counted.
The sync button on the app dashboard refers to syncing your step count with Apple Health or Google Fit. Your step count is automatically updated periodically in the dashboard; you do not need to press the sync button for this.