The app has a feature called Postpone delivery. It gives you 15 minutes extra rental time for free. The function becomes available if you have 10 minutes left of your rental time, and are standing next to a full station. If you have extended rental activated, you will not get this feature.
The Postpone delivery button is displayed on the screen showing how long you've been riding.
The function is available for Android v 2.3.0 or greater, and iPhone v 2.2.1 or greater.
You can read more about the perks of using the app here.