If you need to leave the game for whatever reason, the application will pause. For example, if you get a phone call mid-game, you can answer, and the game will resume when you end the call. The app will continue to run in the background and resumes if you switch back to the app. To save battery life the GPS is turned off when the screen is turned off.
To receive notifications while switching between apps, select the “Also detect a location in background” in the general settings.