By turning the feature on, the online Address Book said it was syncing, and used the information from the last sync done with the iSync application. When it was finished, which was a relatively short amount of time to sync a few hundred contacts, everything showed up in the .Mac Address Book.
The online Address Book gives you several features to control the way your contacts display including how many contacts to display per page; display order (Last name, first name etc.); default email (work or Home); default phone 1 and 2; and the default sort order.
You also have the option to turn the synchronization feature on or off through the online preferences.
According to the .Mac Web site you will need iSync and a valid .Mac account.