The app is designed to work as background task, but each mobile OS platform supports this task differently.
Android: The app keeps tracking in the background until you hit STOP button. You can use other applications at the same time with no impact on the background tracking. You know the app is running in background mode when you see the small app icon in the phone info status bar.
Windows Phone (7.5 +): Make sure to allow the "Run Under Lock Screen feature" in app settings (category Other). When you press LOCK button on your phone the app starts running under lock screen. It keeps tracking, but is not able to sync data in real-time over a live Internet connection. To enable real-time sync you need to bring the app from lock mode back to the phone screen. When other apps are started (you get a phone call for example...) the app stops tracking and you will have to restart it. Windows Phone 8 should fix this issue.