If activated, the synchronization will write the app’s records to a Google Calendar. You will need to configure the calendar in the app’s settings. Every change in Swipetimes is reflected in the Google Calendar, but calendar changes are not transferred back to the app. It’s an one way synchronisation.
Depending on your purchased add-ons you may choose to have automatic synchronisation (happens every time your tracked records change) or you may have to manually invoke the sync.
In order to see the records in the Google Calendar please take a look first by using Google’s web interface. There you are able to also see the calendars Swipetimes created.
The app will use only the calendars it created. None of your other calendars will be touched in any way by Swipetimes.