With the app set to auto sync, the app will:

1. Automatically sync with Remember The Milk when it launches or becomes active (for instance, after the device has been in sleep mode).
2. Wait several seconds after you've made a change, then automatically begin syncing with Remember The Milk.
3. Receive changes from Remember The Milk (e.g. in the web app or other services) after several seconds.
4. Periodically sync around every 5 minutes while the app is open.

When the app syncs, any changes you've made back to Remember The Milk, and checks for any changes from Remember The Milk too. If you don't want to wait for the auto sync, you can manually sync by tapping the sync icon at any time.