Here’s how Facebook Photo Sync works, at least for those lucky enough to be in the test. You go to the Photos app in the Facebook For iOS app’s sidebar navigation or the photos section of your Timeline and scroll to the bottom. There you may find an option to check out Photo Sync and then enable it, as seen in this photo sent in by tipster YoloFahad (an Instagrammer-extraordinaire). Facebook will then automatically upload photos you shoot to an unpublished album only you can see. From there you can authorize which photos you want posted.
More information on Facebook Photo Sync can be found at the link below. It's unclear when this feature will see widespread availability.