You can start and end your booking with the app, but if your phone does not have any reception or if the battery is dead, you can use an alternative access medium - an NFC-enabled card of your choice (e.g. your credit card or electronic train ticket). You can add it in the menu of the app under "My profile → My badges".
To make sure that only you can start the booking, you will have confirm this when making your first reservation by entering a PIN code on the Terminal corresponding to the DD/YY of your date of birth (Ex: 24/05/1995 = 2495). To start or end your booking with the badge, simply hold it over the card reader behind the windscreen.